I'd think it's exactly because they're so powerful, by limiting one thing about the spell you can make it stronger. It would become extremely unbalanced if spellcasters were capable of using Personal buffs on other party members, just imagine being able to use Divine Power on the Rogue/any 3/4 BaB warrior, or Righteous Might on the fighter/barbarian, it'd become really silly really fast.
I suppose that Wizards had been using a sort of point economy system for Spells, much like they did with Classes. So for example, having a spell make someone Large size, adding damage reduction and giving a Con bonus would all substract points from the ammount you have for spells on that level, but applying Personal Range on the spell would offset some of the loss, making it possible to keep it within the point economy.
This part is just speculation on my part of course, but the point remains the same.
Limiting a spell in one aspect = Making it stronger in another.
EDIT: Also, what Veekie said =)