From a numerical standpoint, why would you ever cast the high-drain Increase Reflexes spell at any force higher than 1? Perhaps I'm missing something here, so I'm going to piece together what I'm considering.

1. Spellcasting test is Magic + Spellcasting, unaffected by Force
2. Drain resistance test is based on Force, so lower is always better.
3. Dispelling might be taken into effect, and a low force can make a spell easier to dispel, but if it's only a temporary effect, this probably won't factor.

So what gives? Where does the Force of this spell factor in?

There was a thread somewhere out there answering that.
Well:
Force limits your max. applicable hits (p. 171, Force). Since you need to hit a certain threshold, you need a certain force to get those (unless you use Edge).

They had the problem you mention in the old edition, so it's an easy thing to think.

as a note for dispelling, if you've got the kamra to burn (but really what mage ever has extra karma), a higher force would be good for quickening the spell. This is defintely a good one to have with a permanent effect.

one that wants to have 4 initiative passes to cast spells with =p

if you do a force 6 reflexes, with 12 karma spent, unless you're in a uber powered campaign it's going to win against almost any barrier you may pass through.

not to say that mages don't ever have karma, more that mages are such a karma sink that there's always something they're spedning karma on.