It's funny -- just this week, I was thinking about switching magic over to exactly this system. For two reasons, I decided not to:
1) If I cast a mass command on a group full of PCs, and they all roll saves against it, I'm likely to affect some of them and not affect others. The battle becomes significantly harder for them.
If, on the other hand, I cast mass command on a group full of PCs and then roll to see how well the spell comes off, I stand a decent chance of either affecting the whole lot of them (if I roll well) or affecting none of them (if I roll poorly). If I roll well, the spell can become an instakill for an entire party. Not so much fun.
2) For everyone, it's fun to roll your saving throw to see if you resist a spell. For spellcasters, it's fun to roll your spell's effectiveness to see if it goes off well. Switching from an active to a passive saving throw system gives spellcasters all the fun, and makes nonspellcasters just sit there picking their noses when spells go off.
Daniel