I was reading over the SR4 grimoire. One thing I noticed, it seems to be that force has no bearing on how effective the spell is with the exception of:
- Combat spells, where base damage is equal to the force (plus net caster successes)
- Opposed manipulation spells, where the subject gets to resist every Force rounds

In the case of say detection spells, why would I want to cast Detect Enemies at force 6 (suffering 2 drain) when I can learn Extended Detect Enemies for an additional 24 meters range, suffer drain of 1, and still get the same effect (since results are based off the net successes of the caster)?

Force = Range for AOE
Force = Max Hits on the casting roll
Force = Base Damage on Combat spells

In SR4, total hits are limited by the force of the spell.

So if you have magic 5 and sorcery 5, so you're rolling 10 dice, you can expect about 5 hits with that, and you'd do better to cast at force 5, right? That also drops the drain by a notch (5/2 is rounded to 2, while 6/2 is still 3). Correct?

You're more likely to roll 3-4 hits with 10 dice (1/3 of your dice pool on average). So, in that case, a lot of the time, I'd only cast at Force 4 to get the lowest drain. Actually, you might as well go with 5 if you're rounding down.

That is, if it's the sort of spell where extra hits don't matter. When I'm rolling Increase Strength, I go with 5 (my base Strength is 5, so that's the minimum force required- another reason to cast above force 1), and with 10 dice, I rarely roll more than 5 hits anyway. When I cast Armor, I go with 3, since the drain is actually pretty nasty, and 3 hits (8 dice with non-health spells) is about as good as I can hope for anyway.

Most spells benefit, or at least can benefit from a lot of hits on a spellcasting test, but when it doesn't, you might as well go with force 2 or 3 for the lowest possible drain.

Dread Polack

Also relevant later on is that Force of a spell affects how hard to dispel it is. If you're walking through a ward or an enemy mage is dispelling your magic, you don't want your spell to go pop at the slightest bit of resistance.

This is doubly the case if you're spending some karma on quickening the thing (an initiate power to make spells permanent).