When I rolled my baby UD lock (now 52 and lost somewhere in Feralas), my warlock friends told me that my first five talent points MUST be spent making Corruption instant cast. "Everyone does that," they said.
Well, apparently not everyone. There's a 22-page thread on the General Forums arguing for a trainable instant Corruption. A lot of endgame locks spec 0-21-40, and don't have the talent points to spend in Affliction. But Corruption's spell damage coefficients treat it as an instant DOT, even though it's a two-second cast without talents. Most locks don't consider Corruption worth it without the talents because of this.
The thread quickly degenerates into mage vs. warlock fighting, but it does raise an interesting point. A lot of other "essential" talents have become trainable -- Innervate, Evocation, instant Arcane Explosion. Should the locks get the same benefit, or are their dots already powerful enough?