Quoting myself in a moment of facebook indignation today:
People say “fuck Flarf” for various reasons, I think: 1) a general snottiness regarding ‘the contemporary’ 2) a resentment of the overall good time we seem to be having and our feel-good kumbaya closed-group identification 3) as an expression of received opinions to try to cement solidarity in united disdains 4) as a way to reify their own “gravity” and “seriousness” and hence “importance” 5) because they genuinely don’t like the work, but they have actually read enough of it to earn the right to say so. Of all of these responses, I respect only the last.
Otohime 
Nos 2 and 4 seem to be related, methinks. Why are so many people “anti-fun” when it comes to art?
Nadia, I'm with you except for the “closed group” part of #2. I'm not saying Flarf is a closed group, but in general terms, reaction against closed groups seems to have been justified in the past – the movement against the New Critical academy in the '50s and '60s, for example. Although many of those oriented against the academy were, of course, pretty group-oriented themselves. Putting a foot wrong in the San Francisco Spicer scene seems to have led to instant banishment.
But I'm a little unsure if you mean that Flarf is a closed group, or simply that it's perceived as such by some of its detractors.
you forgot 6), or maybe an extension of 5):
“And what’s happening is a willful dependency on corporate tools to do the searching, selecting, and contextualizing of poetic material, with no intra-textual suspicion or extra-textual analysis of the tool itself or what this means for the ‘product’ that’s being made.”
pulled from essay i assume you've read? http://jacketmagazine.com/29/hoy-flarf.html
That's been addressed by several people, including a very good response by Brian Howe on his blog (google it – ha!).
It all comes down to the fuzzy field of critiquing intentionality, and assumptions regarding the writers awareness of such issues – and, I believe, it is unfair to assume ignorance. I'd far rather assume that Flarf operates in full awareness of such, and is a knowing appropriation, subversion and redeployment of such (much as Howe describes his use of Spellcheckers).