Its generally an ok spell provided its limited to gishes.
Wizard/Fighter/Eldrich knight even with wraithstrike is still going to be worse than a strait wizard or even an optimised melee character because of the strain on their feats.
I'm thinking I agree with you here. Though you can make better gishes than that, it still makes sense that they should be using spells to make sure they hit stuff just as good or better than the Weapon Focus chain boys.
They "generally" lack the ability to take 10(or 11) on attacks so the whole thing they do is buffing.
I wouldn't strip them of the best buffs available across the board I mean whats the next best thing? It can't be true strike? and polymorph is better cause it does so much more than just help you hit things.
Saying that something is an "Auto-include" for any X build isn't really that negative a mark. I don't like this argument...
Its kinda like, yeah so what? So is power attack for Str based builds, so is improved natural attack for Natural attack builds "Monks,Totemists, psywars...oh my" Arcane Strike is for duskblades. Point being pretty much every archtype has something thats a "auto-choice" and thats not a bad thing, it makes sense, for them to take whatever fits the concept mechanically the best.
So no, you shouldn't BAN wraith strike, but it should be usable as a spell. As a spell it cost you something, persisted or no. Persistant wraithstrike should be a 9th level spell slot. . . thats not even all that good for a 9th level spell slot, and 2 feats.
As an item that the whole party has? No. Never.