It's way more likely that somebody lets you use their words, and that they get published unedited, than that they let you use their likeness on your website. So I don't find it dubious at all. I guess I'm naive or something.
But seriously, if I were doing a Y Combinator application, I would make sure that my testimonials were spot on. I would do so because one of Y Combinators' most enduring bits of advice is 'build something people want.'
I'd even equate that to grammatical errors in a website. You're competing for an interview against thousands of the best minds around. Proofread your copy!
Rather, I'm saying that when you apply to Y Combinator, you are competing against the best entrepreneurs around for a very limited number of seats. Since Y Combinator is so competitive, your chances of getting in would be better if you started working with a good writer or editor.