Clipping missiles and increasing your cast time on arcane blast turns out to be pretty bad for your damage, particularly the missiles. Any interruption loses you a big chunk of damage on that spell because you just don't get the missiles off.
My guild was working on Rotface and now Putricide, where there's a lot of raid damage, lots of things like expanding slime pools, damaging auras and explosions. Would it be better to not take any damage? Yup. Do I expect that to happen any time soon? Nope.
Also, as pointed out in the EJ thread, the common spec of 3 points in pushback reduction (60%) plus a non-healer Paladin concentration aura (35%) doesn't fix the missile clipping problem as any clipping will lose you a set of missiles.
Does it help? Well I don't need Slow on Putricide and I don't seem to have range problems. Of course, Putricide isn't down yet (had him at 200k on one wipe...arg), maybe I'll try something else next week. I don't have numbers, but I'd estimate I'm getting ~5% increase *at most* from this.