Some ideas: raiding, seiging, spell AI and more..

[Soapyfrog]

Arryn said:
Of course the same argument could be said about only summoning Bane Lords with your death gems, or Vine Ogres with nature gems (well, Fairy Queens are very nice too).

Yes its true there are optimal summons for all paths. however you do frequently summon other stuff too, and thats useful too... sometimes you will summon wraith lords, or demi-liches, or lamia queens. You will cast arouse hunger and ghost riders and call of the wild, etc etc. There are many useful things to put your nature and and death gems (well nature gems mostly go into fetishes if you are diligent, but still.)

The point is that while there are optimizations, they dont invalidate other strategies in the way that, say, soul contracts do. You cant compete with a soul contract in terms of regular units with any other blood summons. At least in the other arenas, the optimizations are not so blindingly obvious...