Healers can only heal damage you take, tanks on the other hand reduce the amount of damage you take by a lot.
Both are very important, but because tanks dont generate a big number telling you how much damage they blocked they are usually ignored
Usually I see republic win. Also every angsty teen wants to play sith so you really shouldnt be so surprised that the empire has such a large population. Also I disagree on the gear issue. Republic has far better gear, but thats just my opinion