I'm a fan of Dillin's work--particularly his later JLA stuff--and have a few of his pieces in my collection. He may not be my all time favorite artist, but one I definitely appreciate.
For me, part of it is admittedly the nostalgia factor--he was the artist of some of my favorite JLA stories of all time. If we all look at some work with rose-colored glasses, Dillin is definitely one of those for me.
Another part is that I genuinely appreciate how he organized his panels and pages. Team books are a huge challenge that way--lots of characters to include, make identifiable, and make interesting--and I do think Dillin--especially his later work--managed those aspects well. He could crowd a dozen characters into a page or a panel and generally managed to make 'em interesting comics reading. Maybe not ground-breaking, but generally interesting. (I like Schaffenburger, too, but I think he has a different energy than Dillin did...) I understand why he's not everyone's cup of tea, but I do appreciate the skills he did bring to bear.
I also remember reading in the JLA lettercolumn when I was a kid that he had died unexpectedly. It was a very specific moment in my young comics-reading where I became really, truly aware that there were actual people writing and drawing and creating these superhero fantasy stories, and that surely has something to do with my lasting admiration of his work.
Not sure that I've noticed more Dillin pages coming to market, but I have noticed prices (asking prices, at least) creeping up on Dillin pages, so it seems more collectors are coming to his work of late...