Dane, maybe I'm an exception to the rule and hopefully it's the result of having other (non-snow/alpine) climbing experience, but I would never assume that a summit is guaranteed. If nothing else, as an "old guy" I know better than to think that the mountains always deliver - guide or no guide.
If we're talking specifically about guided summit trips and folks with near-zero mountain experiences, then I can see how the pressure might be pretty intense on guides to deliver the experience.
I don't envy the guide services and schools. OTOH, for someone with limited time and not living near such mountain ranges, a "school" was an excellent vehicle for acquiring some essential glacier skills.
Anyway, I just wanted to comment on my experience - and it's true that I was on a different peak, and under diff circumstances. YMMV...