I personally think it's because of PvP.
It's castable in combat, with about 1/2 to 3/4 of both bars filled.
Plenty of times in group vs. group PvP, I raise the rogue or shammy, and they turn the tide. It's basically an extra person on your team.
If I could cast it at will... well, I could raise someone every time I popped a mana pot, about once a minute. I could raise 2 or 3 people after using Innervate. All at a decent combat potential... during mid fight.
I also suppose you could make an arguement about PvE... Mage out of mana, mage suicides, you raise him, giving him almost as much MP as you spent.... making Rebirth a "poor man's Innervate", I suppose.
Idk... I just know it's annoying, spending it on random corpses I see laying around... and finding they logged off after they died ;_;