the same could be said for keeping the upgrades separate. why spend 3 times as much TP/MP for an upgraded version when the tier1 version will make 1 turn quick fight of a random encounter and not even phase your ability points?
I'm not really sure what you're asking about, but I'll try to address all the possibilities since you weren't very specific, especially after quoting my post, which is a specific example of my RPG.
In almost every RPG I've ever played, MP is the absolute most renewable resource in the game. It regenerates on its own, the dev has sprinkled MP healing items everywhere in the game, Inns restore MP for a fraction of the cost of a consumable, spell costs are so low as to be nearly pointless, etcetera. So the "using TP/MP" thing is basically a non-issue unless you have no idea how to heal up after combat and are a newbie to RPGs (which, I guess some people are, but most really aren't, and they'll hit the next two towns on the journey before needing to recover their MP again, unless they've been grinding enemies for levels, in which case... they should come prepared).
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If you're asking about simply "I get more spells and keep my old ones", that's the argument about point 1. If you've got Firaga, how often do you ever bother to use Fire? I'd wager "none" since anything weak enough to be killed by Fire at that point, is weak enough to be killed by expending 0 MP and by just mashing "Attack". What you end up with is this large list of Skills a player doesn't use all that much. A large list which they then have to scroll through to get to the spells they want to use. I want to cast Firaja, but I have to scroll past Fire, Ice, Lightning, Poison, Fira, Blizzara, Lightning 2 (for the life of me, I can't remember the stupid FF names for these spells), Poison 2, Fire 3, Ice 3, Lightning 3, Poison 3, and oop, right there, there's my Firaja. I had to scroll past a whole bunch of spells I no longer use... just to get to the one I do use. Two great examples of this in Final Fantasy are FF6 and FFX. Or maybe FF9 and FF7 as well. pretty much anything after FF5, to be quite honest.
Maybe this is design philosophy you don't agree with. But, some devs, like myself, believe that every skill should always be useful (as in, for every single fight, including the end boss fight) or older skills should be absolutely replaced by newer ones in order to cut down on menu clutter.
My solution was to attach States, buffs, etcetera to my Skills, use percentages, and to limit myself to 6 or up to 9 skills a character. It is, by no means, the only solution.
But, you should go through every RPG you've ever played and try to tally up how many times you used "low level" skills once you got their next iteration. How many times did you use Fire once you got Fira? How many times did you use Fire and Fira once you got Firaga? How many times did you use Fire, Fira, and Firaga once you got Firaja? How many times did you use ANY elemental skill once you got Ultima/Flare/Meteor?
How many times did you use Lightning in Chrono Trigger once you got Lightning 2? How many times did you use either of them once you got Luminaire?
I think you'll find the answer is somewhere along the lines of "rarely". Or maybe, "only when I was low on MP and couldn't cast my big spells".
While you're at it, go ahead and go through every RPG you've ever played and try to count up the amount of times you've ever "run out of MP" or didn't cast a spell because "you didn't have enough MP". I'd wager that amount is pretty low as well.
It's one of the reasons the White Mage (or Dedicated Healer) is always in the player's party. MP is so plentiful and healing with Skills is more cost effective than buying Potions in town.
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If you're asking about my specific combat system then... here ya go:
My combat system doesn't revolve around "one hit kill random encounters". It is balanced for "four hits to kill a single enemy". Does that mean you can gain a bunch of stats and come back to beat up on those monsters in a single hit? Yes, you absolutely can. But, level ups in my game don't grant stats. So, the only way to gain your stats is to keep moving forward, keep buying equipment, keep completing quests. You gain stat points via Quest Completion. So, it's fairly easy for me to balance the "four hit kill" rule across most every random encounter.
The rule, however, has exceptions. The first time you encounter a monster, you are meant to take at least 4 hits (one move for each party member) to figure out how to kill the enemy. If you figure out its weaknesses quickly, then you can kill it in less than 4 hits. Maybe you discover its weak to piercing damage. Maybe you discover it's also weak to ice damage. Maybe you discover it can be easily put to sleep. Your next encounter will go much more smoothly after that. Those four hits could turn into two. Maybe even into one if you are clever. But, the rule is "four hits to kill".
But, my combat system isn't about "ending combat quickly". It's about combat teaching you things. The random encounters teach you mechanics and combat concepts. The boss fights are the tests on those things. I dislike games that don't treat combat in this way, so I am making a game that treats combat in this way.
As for the reason you'd want to spend as much... It's because they get more powerful. If you'd like a little insight... a peek under the hood, I'll give you an example.
Fire (Tier 1).
MP Cost: 3
a.mat +10 - b.mdf
50% Chance of inflicting Level 1 Burn.
Level 1 Burn removes 3% of your HP a turn for 3 to 5 turns and removes 3% of your Defense stat.
Fire (Tier 2, Effect Path)
MP Cost: 6
a.mat + 20 - b.mdf
60% Chance of inflicting Level 2 Burn.
Level 2 Burn removes 5% of your HP a turn for 3 to 6 turns and removes 5% of your Defense stat.
Fire (Tier 2, Power Path)
MP Cost: 6
a.mat + 30 - b.mdf
60% Chance of inflicting Level 1 Burn.
Refer to the above Level 1 Burn description.
Immediately, your MP usage doubles, but you're getting something useful out of it. Higher levels of states. Higher damage output. Higher rates of inflicting states. It inflates wildly after Tier 2 and into Tier 3 and even Tier 4 (Burn goes up to Level 4 with 20% HP removal a turn for 3 to 10 turns and 20% Defense stat down. Attack power goes up to a whopping +100 bonus to your Skill. You can also "mix and match" to an extent).
The skill is "tactical" in nature. It isn't about pure damage output, because if you just want to "one shot" a low level enemy, mash "Attack" or use my "Zero MP Cost" skills that do the same thing as Attack with a different element. Final MP Cost ends up being 50 a cast, but by the time you reach that far, you'll have a decent enough pool of MP that it's only a restriction of builds.
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Hope that helps.