Trade in Void staff for more AP? Hmm no.
Trade in Void staff for more Utility? Yes definitely!
Void staff is a pure damage Item. The item is usually purchased because Magic resist Items are pretty common in the current meta (especially GA and Banshees). The Void Staff itself is becoming really effective after an opponent has over 100 MR, since yours spells will then deal damage as if he has only 65 MR. It increases your damage over all, even with opponents that don't have MR items.
Now as already said, Void staff is a really important Item for every AP Caster and you should buy it on everyone who deals mainly magical damage. You can however wait with buying it. With most (not all!) AP champions your Item path should be -> Core Item (different from champ to champ), Boots, Rabadons, Utility Item, Void Staff, Situational offensive/defensive item.
For Annie I'd say a good generic path would be: Rod of Ages/Deathfire Grasp -> Boots -> Rabadons -> Zhonyas -> Void Staff -> Abyssal
Of course you shouldn't build like that every game but It's a safe build if you don't fall behind too hard. Also if you see that you won't deal any damage because the enemy team is stacking MR, Get the void staff earlier.
The most Important thing about void staff (and rabadons aswell) is that these Items scale with AP. If you rush them too early they won't be as effective as other Items. This is why you usually get Rabadons As a Second AP Item and Voidstaff as a 4th or sometimes 3rd or 5th.
Also a thing to mention: If your opponents really build no MR at all (which is rarely the case), you can also rush a Liandrys Torment/Abyssal. Different from % Penetration, Flat penetration/Flat MR Reduction can really put you ahead early, even without that much AP.