So I learned not to bring out the priests too soon as it can end up feeding your opponent.  There are two problems with this element of the game though as a mechanic, and I think I can suggest a good solution, but first the problems as I see them:

1.  People tend to wait so long to summon priests that you can easily get priests, angels, catapults in one pass.  I see this more often than not when both sides consist of good players and I personally think it is kind of lame since essentially the extra creeps are not seeing much (or maybe even intended) use in high skill level games.  A bit more on intended use in the last paragraph.

2.  Even if you know not to summon priests, one person who doesn't get it (and isn't paying much attention to chat or doesn't care) can throw the game for you by summoning priests and then not pushing towers with you.  Quite a downer.

Possible solution: Replace the position of Priests and Angels, and possibly make angels worth less experience or just release one at a time per creep wave.  This would be good for two reasons I can see: one, angels do a lot really such as increasing vision radius, yet they rarely get used for this reason since you buy your units all at once late in the game.  They mostly serve right now as something you have to buy before you can get catapults, since they don't add much damage.  At which point the extra vision isn't helping you since one side is in the other's base.  Two, the time when the extra vision would help you is when the game is still in an early stage and neither side is on the defensive, since that's when you want to have a wider field of vision.  So switching them would in my mind not only improve the variety of creep usage in games and stop bad players from feeding opponents with priests, but would also improve the angels and perhaps get them used as I would think was intended more often.