Counteracted by the optional rule under Dice Pool Modifiers, pg. 61, SR4A.
In fact in this case it won't necessarily be a good idea even though I will stick with it.
Just have a look, if we discard material things if someone want to summon lets say a force 20 spirit, the dice pool of the spirit for binding will be lets say 20, far from 40 normally. Well, with successes upon a 20 dice roll, the drain should be harmfull...
It does concern me that street magic explicitly excludes dice gained from an ally from the limit of one bound spirit performing aid sorcery per test. So 10 force 6 ally spirits would add 60 dice to a spellcasting test... makes a power focus seem obsolete.
An Ally costs 8 karma per force and can add force die to all your spellcasting tests.
A power focus costs 8 karma per force and can add force die to all your spellcasting tests.
And you can have more than one ally? Ouch.
This is the stuff of campaign villains ;P
Here I'm more concerned about ally spirit with possession (I really thought that only possession tradition can summon a possession ally spirit). Lets give a try to a force 12 ally spirit possessing it's caster, with also spellcasting skill. The caster know have 12 in its magic attribute, 12 in its spellcasting skill and 12 in its aid sorcery

Not bad you will say

I have also another point, I thought until just a minute ago that aid sorcery affect only Sorcery skills, not Conjuring ones (as they can't use conjuring skills). I just saw this :
"If the ally spirit attempts to break free, it must make an Opposed Test pitting its Force x 2 against the magician’s Magic + Binding (the ally does not need to add dice to the master on this test)." Loosing an ally SM p105.
Assuming that you use an attribute of magic bound to the force of your ally, aid sorcery, a binding skill and a focus (power)... it will be easy to bind rapidly really huge ally spirit (even easier with the limitation upon dice pool).
Drain you will say... Yes, the only problem with multiple ally, you can just sacrifice one for a really huge and nasty drain. Not to say throw manabolt at 20+ force with no drain

Well I stop my Villains Bad Guy dream as I won't allow something like this in my campaign

And the NPC I try to build will be a good guy

BA