Go to Alan Baker's blog, Litterbug, and scroll down to the post for September 21st (there are no titles). He makes what is, I think, a very good point, and one that I touched on earlier in the year - the fact that poetry collections seem to be getting longer, sometimes at the expense of quality, rather in the same way that the advent of the CD encouraged bands to fill every album with 70 minutes worth of music, regardless of whether it was worth recording in the first place.
Of course, it's not always true, and there are plenty of collections of which I'm glad that the poet kept going well beyond the old 64-page mark. But Alan's point still stands, I think. In such times, chapbooks can offer the perfect format for a writer who wants to get work out there in between increasingly chunky full collections.