I think there are a few ways the Demons could be used.
1) Summon the Demon. It creates its own ectoplasmic body, which it presumably has to expend Will to maintain, which means it has trouble with Thresholds and such.
2) Summon the Demon INTO an Ectoplasmic Shell you have created using a separate Thaumaturgic Ritual (in addition to the Circle, the Summoning and the Binding). Now the only logical advantages I can see with this is that:
a) the Demon is presumably expending less energy to occupy the Ectoplasmic Shell than to maintain an ectoplasmic body it self-generated - is this a power boost? Or is it more like item c (see below)? How much of a power savings should it be?
b) the spellcaster has some control over what the Ectoplasmic body looks like (of debatable permanence and utility - I wouldn't imagine for a second that Demon can't warp the Ectoplasmic Shell for its own purposes)
c) the Demon may be able to use the Ectoplasmic Shell to minimize the hazards of Thresholds - maybe the Demon gets to Invoke its Ectoplasmic Shell to add to a defense roll against the Threshold, or maybe the spellcaster would build it to negate X ranks of Threshold - seems like potential for abuse, but it is a potential advantage
d) the spellcaster may be able to give the Demon a particular power it may not possess when generating its own ectoplasmic body
3) Summon the Demon into a Golem Body of some sort, like the Demonic Co-Pilot ability in the Powers section. Presumably you build the Golem Body with particular attributes and abilities, then "enlist" the Demon to "pilot" it. That means you could build either a Diamond Golem that shoots sunlight, a Clay Golem with Malleability, or a Coal Golem that can shoot Brimstone, and whichever you built, you could theoretically enlist the same Demon to pilot it. Or the next one.
4) Summon the Demon into a Mortal Body. This sounds at one end of the scale like a Plot Device, and at the lower end of the scale, the before-mentioned Demonic Co-Pilot.
The Demon would, of course, want to be used instead of an AI so that it could wreak havoc and corruption here in the material world, and it would provide benefits up-front to entice a would-be Summoner. So however we balance things out, it should always be cheaper and/or more effective (in the short term) to rely on a Demon (but ultimately less safe).
I like your observation about AIs probably building off of the spellcaster's will. Would the AI then be limited by the spellcaster in the skills it could have? Would someone with a Mediocre Burglary be able/unable to craft an Infiltrator Golem and stock it with a Superb Burglary?
Can you imagine listening close to that field of conjured frogs and realize they aren't actually croaking, but saying "Ribbit" in Harry Dresden's voice?