I think it partially depends on other factors in your battle system. If you have the kind of game where damage formulas are transparent and the player is highly focused on stats and numbers, then you'd BETTER show those bars, and possibly even the numeric HP value. This is the case in most strategy games. And even then, scanning or having those numbers from the start is also a decision that varies with the style of the game. If the player Isn't aware of these things and the game is supposed to be more imersive; like in a horror game, for example; then having bars can break the immersion and suspense a little.
I personally find long, drawn out battles that give you absolutely no indication of how much progress you're making to be incredibly frustrating.
There are more options than simply using HP bars or scanning stats.
One solution I like, is visible damage on the enemy. It can be resource intensive, but if you have an alternate battler graphic at different levels of HP then it shows players how much progress they're making without an obtrusive bar. Visual indicators like visible scratches/injuries, armor breaking and falling off, etc. Or just having enemies color change (A common one in some games is applying a red hue at low HP)
But I think we're starting to derail the original question, because I get the feeling from the opening post that JAD94 is already planning to use bars and is just trying to decide if you should have to scan for them first or not, so debating to have bars at all is slightly off topic. If scanning is something that you always have to do in every battle, against every enemy, it can be tedious to the player. If it's something you only have to do to a new type of generic enemy the first time you encounter it, it can kind of add to the feeling of exploration and discovery, especially if you use a scan script that shows you a 'codec' for the new creature you scan, or shows you some new stats.
Another possibility to make scanning feel more useful and less tedious is making it a viable skill that does more than just show information. Like scanning an enemy might debuff it's accuracy, because you now understand it's attack patterns which makes it's skills easier to dodge. Or scanning giving you skills or items or whatever based on things you 'learned' from the scan. That could work especially well for a sci-fi game where there's a scientist or researcher somewhere that reads the data from your scans and gives you new skills and equipment based on what they learned.
"Hmm... based on this data you sent me on dragons, I was able to develop this new armor based on the fire-repelling properties of their scales" That sort of thing.
So basically, what I'm saying is that if you have a scanning mechanic, you might want to consider jazzing it up to make it more interesting to use.