Because then your sneak attack is always fire damage, and you are damned to hell if you face a red wyrm. But hey, that is one way to work around missing. Also, pyrokneticist doesn't seems to be able to TWF.
Hitting stuff is not a rogue problem, everyone but the casters will need solid investment to go past creatures AC after a while, its expected you find ways to hit stuff. TWF needs a bit more of investment on this than using a two hander. But the guy using a two hander, unless abusing shock trooper, needs heavy investment on to-hit too, or else he can't power attack at all.
And if you can't hit your power attack, there is no damage, or so I hear anyway.
But indeed, resolving stuff as touch attacks is a nice way to do it, I just didn't want to make custom item of wraithstrike. That would be cheating =p