well, in general, I'd do that as a courtesy. However, there's always 2 sides to the story, and in my experience, many people asks for a "favor" after they asked for several free meals already, so not sure what's the deal here.
Also, seeing you seem to have a very special luck with designers maybe there's something wrong with you instead of designers. I really don't know, but the first thing that jumps to my mind is the words "client from hell". Dunno if this is the case, but if so, I can understand why someone isn't bothering too much to give you something for free.
And by the way, I spent tens of thousands of dollars at the supermarket, and they still don't give me a free pack of Doritos, go figure...
