Point-for-point answers?
1) Yes. I've used mini-bosses for each of my games, and will keep doing so.
2) Depends on the area. I prefer mini-bosses that fit the dungeon or adventure -- mutated hero in a warlock's stronghold, haunted armor suits in a painted castle world, goblin with magic in their forest, sand worm in a desert, et cetera.
3) Serious answer? Provide a halfway challenge to test the players' mettle, and to up the ante compared to throw-away encounters. Some could even drop equipment you can't find otherwise in a post-battle event.
Silly answer? To have a reason to include another boss theme, preferably one sharing the main fight theme's motifs. X3
4) Yes and no. Some optional bosses can be as tough as mini-bosses, while others can be superbosses that surpass normal boss fights. But for me, most mini-bosses shouldn't be optional.
5) Mini-bosses should be somewhere between trash mobs and normal bosses in difficulty and length of fight. From my POV, throw-away battles should be won in about two rounds if you know what you're doing. Bosses should take eight or more rounds, maybe less if they're meant to deal serious damage quickly. Thus, a mini-boss should be beaten in maybe five to seven rounds? Maybe? IDK.
Also, one thing I like to do is give bosses "pinch modes" at half health or less. This is an evented action that changes the battle a bit -- a new buff on them (or debuff on your party), a powerful move used immediately, maybe even a countdown to instant defeat (or rounds to survive for an immediate victory). Most mini-bosses in a dungeon shouldn't have these, but mini-bosses that are optional might do well with them?