I see what you're saying and I agree on all points except one.
If you (multi)cast 2 spells at force 6 each, it does not do 12S damage. It does 6S x2, each resisted separately by the target thereby giving them a better chance to take less damage. On average, they will reduce the damage taken by double the amount you're assuming they will resist from a single cast. Which brings up the question of "why are we assuming only 1 hit to resist?" A really basic starting character is going to have at least 3 willpower and most likely 3 or more counterspelling dice allocated, which should net at least 2 hits on average - and that's a mundane. If you're target is a mage, you'd be lucky if they have only 9 dice to resist with which will net 3 on average, usually 4 when accounting for real life rolls.
Also, as a side note - you make your Spellcasting + Magic Roll first, deciding how many of those hits to allocate to adding to the DV and how many to set aside for drain resistance. Then, after you've already decided, is when they roll to resist. So if you set back all hits (or all but one) you may actually be doing very little damage with the 2 lower powered spells. So in that case, even though you are taking far less drain, you're also doing far less damage.
I suppose at this point it's just a matter of semantics, since we're looking at a comparison between over casting a spell vs multicasting the same spell at lower force, because as you said, you'd have to be dumb to use the extra hits for damage on a single spell with the optional rule in place, but I really don't agree that said optional rule encourages multi-casting. Overcasting probably, but not multicasting. By multicasting lower force spells all you're doing is giving up damage potential to reduce drain.