Jeff VanderMeer has a blog post about closed vs. open anthologies in response to Jay Lake's post on the same topic. (Ellen Datlow joins in the discussion at Jay's livejournal, as do some others whose names might be familiar.)
Now, as I'm at a stage where I'm not getting invited to closed anthologies, obviously I like open submissions...but that's simply selfish reasoning and should be taken with its own grain of salt. I do like Jeff's reasoning--including his caveats of when it does make sense to keep in antho closed.
There's one thing that I wonder if it needs to be thrown into the discussion, and that's the question of whether having an open submission increases awareness of the antho and therefore sales. A question is all it is at this point--I'm not saying that it does. But I know a number of aspiring and beginning writers get most or all their market knowledge from two places--hearing about it from another writer at a similar stage as themselves, and Ralan's. I try to be a bit more aware, with Nightshade forums and some author and editor blogs, but I have to admit that's where I get a lot of my info too. So if an anthology never appears on Ralan's, it seriously decreases the chances I'll learn about it.
This same things applies to magazines and ezines--I'm much more likely to pay attention to Strange Horizons and Clarkesworld (and soon, Fantasy) than to Subterranean Online--and that is actually what got me thinking about this question. I do try to remember to check out the new issues of Subterranean, and the fact that it's only quarterly hurts it in that respect, I think. Though it does mean that when I remember, I probably haven't missed an entire issue or anything. But I tend to only remember to check it when I see a link from someone's blog that they have a story there or if a fellow writer recommends a story from it, whereas Strange Horizons is always one of the first things I look at Monday morning.
So, I'm positive that having an open anthology increases awareness among certain groups of aspiring writers (and that pool of aspiring writers is very large). What I don't know is if that really translates into any difference in sales, or are the majority of those aspiring writers not likely to have the money to buy the anthology anyway? Are aspiring writers, as I've seen someone argue on a certain forum, too tight-fisted and miserly to bother attracting? There may be a slight risk that someone who's been rejected gets bitter and doesn't want to buy the product...but I'd think (again, just a hunch with no evidence to back it up) that it's more than countered by the much greater awareness.
Jeff's post focuses on the artistic benefit and whole-field benefit of open anthologies. This is more on the business side to add to the pro/con tally when considering the question from that perspective. It may not fully counter-balance the question of time required to read slush for a given anthology, but it at least can possibly mitigate it somewhat, maybe enough that the other questions from an artistic standpoint can take over.