Staffs have better stats than two wands of equal level. The idea is that you'd be casting spells instead of attacking with your weapon, with the Staff of Magus ability being more of a backup than a primary. Also, Staff of Magus does not have a durability cost, as attacking with a wand does. If Staff of Magus had unlimited use, there'd be little reason to use wands.
I put a lot of points into Memory so I can get more spells. Through most of act 1, I'm using wands because I just don't have enough spells to exhaust my AP each turn, but later on I'm barely ever attacking, so a staff would be a better choice for the stats (assuming I've gotten one that has better stats than two wands, which is generally the case). Although, part of the problem I have with switching to a staff is finding a good one that does air damage, because I need that to keep groups of enemies in stunlock (electrifying puddles).