It seems in all the pro replays videos that I've watched they don't have unit health bars displayed at all times and instead press Alt to see them. Why is that? I find it invaluable information to see at all times and having to press an extra button to see it takes extra mental energy to consciously think of doing it that could have been better spent keeping other things in mind. What disadvantage is there to having them displayed at all times?
Actually, I'd say it's the shoutcasters turning off the health bars. You never see replays of the gamer himself because that could cost him performance. Those games are either frapsed live through spectators or they just comment on the replay. So you never actually see the player's settings in a game.
Any pro-gamer worth his salt will tell you that you should have the health bars on at all times.
It makes it much easier to focus targets that are already low and makes it much easier for you to see what unit needs to be pulled back. And even in macro game the health bars still stick perfectly to the units, so you can still see very well (I'd argue that it's easier with bars on than without) what's going on on the battle field.
Not really an answer but I find this to be annoying too. In games like Heroes of Newerth, they managed to make the health bars clean and more or less clutter-free. No reason for SC2 not to, but they do anyway.
The stuff posters said about clutter is far more applicable in large-scale RTS games like SC2, though, and it makes this game way harder to micro for.
As far as I'm concerned this is a UI oversight that Blizzard needs to figure out.
Because if you have twenty units, your opponent has twenty units and those two mods clash: how well do you think you can still see the independent health bars? Since they will most likely overlap each other.
Also, most of the time they know which units will get killed, so they don't need the health bars to tell
Example from Ars Technica: I don't find this very useful... And the colors below already show me which unit is dying.
Side-note: It might actually be detrimental for your performance, since it would have to update a whole lot of health bars very very fast, so unless you have a low ping and a fast computer, I'd not use it with large groups