The thing that's confusing about the Destro filler nuke is that the tree buffs so many different filler spells (hello, Imp SB, Imp SF, Fire and Brimstone) that it's completely confusing which talents are intended for Destro and which ones are intended to be picked up by other trees. Is PvE Demo supposed to skip cremation for Emberstorm and Imp SF and use it as a filler? If that's the case then why even bother introducing Demonbolt? With the talents so shallow in the tree it seems to imply that's the design. I can see Fire and Brimstone being reworked to make SF filler for destro what with Afterburn buffing the spell as well; but the desirability of a 4 sec cast time reduction on a 6 second spell makes it automatically seem to scale better due to coefficients than almost any other spell available to warlocks, which means that almost every spec (certainly demo, if not affli) will want to be using these talents so they can use SF as a filler as well.
All in all, my guess is that Emberstorm, and possibly Imp SF will be moved to deeper into the tree and the spells that buff incin will either be removed (making Incin aimed primarily at Demo to be used under MC and possibly for when Backlash procs if talented into by pve'ers) or changed to include SF. Basically with NINE talents buffing SF in destro (12 if you include that backlash effects it as well) compared to the five or so talents that buff Incin it seems that yes, Destro ought to be using SF come cata. Still, I think the trees are really a mess with all of the SF talents at the bottom, given that Locks don't even learn it until 48 (though that could change, of course) and Blizzard wants the early talents to be useful to low-level players.