Yep. Absolutely. I don’t do that as much now, but that’s because I’ve made a concerted effort not to. I think there are plenty of times when agents request material from queries or even from a proposal that they aren’t sure about. We know the storyline is tired or the voice seems off, but the idea intrigues us; we know we’ll probably reject it, but there’s something about it that makes us curious enough to ask for more.
Agents are readers and as readers we’re curious people. The problem is that this is also our job and, like in any job, we need to be very careful about how we manage our time. That means being somewhat sure about what we’re requesting when we request it.
That being said, there are plenty of manuscripts I’ve read all the way through when I knew by page 50 it was a rejection. There are even times I rejected a book, but finished it anyway. It’s the natural curiosity of a reader.