What would be a good encouragement for magicians to cast spells on allies? I mean, wasn't the original rationale of the -2 per sustained spell something like the magician can make you faster, you hotter, and the magician invisible, then hangs around until it's time to drop all those spells and fling Ball Lightning around?
Also, how about this for a house rule in 5th: No Power Foci, but a Spell Focus can work on Spellcasting, Counterspelling, or Sustaining, just not all at the same time?
So you won't need a bunch of foci and risk developing a dependency on them, and your Combat Focus would never get used for sustaining anything, but you could, say, use your Illusion Focus to help you cast your illusion, counter another illusion, or sustain an illusion you didn't use it to help you cast in the first place, but once it's sustaining something you can't use it for anything else.
As for spirits, what about spirits disliking hanging around places that aren't for them? Like, air spirits won't like it if you summon them indoors, People Spirits hate it if someone brings them to the wilderness, stuff like that? Though this would mean that water spirits might see the least use, there might not be any problem with Task Spirits, and this is still a fluff solution to crunch problems, which rarely goes over well.