Weasel Op said:Then what would be the problem with the camera rotating? (yeah I'm back already)
The unit would also look like it was rotating.
Note how camera view goes form near isometric to top down when you zoom out. Well, with a billboard, from top down, the unit would still appear in isometric... kind of like it was lying on it's side.
Billboards are usually used for effects like fireballs and smoke, because they are relatively speherical, and no matter what the angle, they look the same.
There are also other types of billboards, that hold their position, used for things like laser beams... but as you would zoom or rotate, their flatness would be come very noticeable.
The only way to use billboards for units, and have them look good would be a locked camera view.