Just as a side note: I'm neither DMing, nor playing a magician; and the only PC magician, that is routinely around (there is one more, who isn't around often, however), cannot even summon spirits at all.

This is mostly meant to limit the NPCs, if anything (and it was the DM's own observation, that spirits are pretty powerful; especially against mundanes, of course, because their high hardened armor makes it next to impossible to hurt them at all, once you go beyond Force 5 or so).
My first comment was to actually have them roll the summoning and suffer the Drain (we use a house rule, that overcasting gives double Drain, once physical and once mental; and (for spells, anyways) Force is not halved beyond Magic, which means Drain from overcasting spells is based on F-M+M/2(rd dwn) instead of F/2(rd dwn), which is still used for spells within the Magic limit, though).
The second comment was then, if he still thinks this might be a bit much, why not limit their maximum Force to the summoner's Magic Attribute (as detailed above).
Bye
Thanee