Publish and Perish

Up on literary agent Nathan Bransford’s blog there’s a discussion raging over what the optimum rate of production is for contemporary writers. One book a year? Two? A book a decade? One good book a generation?
Bransford’s debate centers on fiction, but it’s quite applicable to poetry as well. The terms, though, seem quite different.

On the one hand, you have poets with long-standing relationships with publishers and editors, poets who have been putting out new books regularly for decades (I’m thinking of poets like Jorie Graham, Mark Strand, Mary Oliver, Charles Simic, etc.). They seem to be on a pretty steady schedule of one book every two years or so, and everyone seems happy enough about it.
On the other hand, you’ve got free-agent poets—some younger and unknown, others mid-career and well regarded--shopping multiple manuscripts around to contests and small publishers, often for years, plugging away at an oeuvre in relative obscurity. The poems pile up, the rejection letters too, until the lucky day comes and then he or she has multiple books coming out in a relatively short amount of time (I’m thinking of a few poets whose work I admire—Eleni Sikelianos, who had four books come out in as many years, and Graham Foust who had his first two books appear the same season).
This creates feast or famine schedules, with some poets putting out books at a rate that would put the prolific William Carlos Williams to shame, and others disappearing for years (Laura Jensen? David Berman?) On top of all of this, if you add the pressure to publish in academia (which I only know of through hearsay), and a marketplace that is lukewarm at best to new products, then the picture becomes quite complicated.
It's a good question, though, for writers, readers, and publishers: how much is enough?