It might indeed be the case that many "negative" reviews of fiction are glib and opportunistic exercises in what has come to be called snark. Such reviews make little real effort to adequately describe the ambitions and strategies of the book under review, or to assess the book's success or relative failure to fulfill these ambitions with any degree of specificity. If this sort of review were to disappear, neither literature nor literary criticism would register the loss.
But to dismiss all critical evaluation that reaches unfavorable conclusions as illegitimate, somehow an affront to the "community" of writers and readers who value books at a time when reading seems endangered, is not just irrational but in fact a greater danger to sustaining a serious literary culture than indiscriminate snark would ever be. That some reviews elevate shallow and superficial pronouncements over considered judgment does not mean negative reviews that are carefully considered should be equally rejected. Moreover, writers themselves should welcome the latter sort of review, since it signifies the work has been taken seriously and given close attention. This seems to me an infinitely preferable response than empty praise, which, in the absence of the more rigorous standards implied by the possibility of a negative judgment, is all that reflexive approval is destined to become.
I wouldn't necessarily use the word "smarm" to identify this kind of praise, although anyone who writes book reviews simply to uncritically celebrate otherwise mediocre writers because all writers need "support" is hard to take seriously. Especially when the reviewer is him/herself also primarily a fiction writer, readers are well-justified to think that such reviews are a form of self-protection at the least, but even when they are sincerely intended, more often than not the lack of real critical engagement with the book under review undermines credibility. Synopsis substitutes for analysis, and judgment is replaced with rhetorical flash and vacuous accolades. Indeed, this sort of rah-rah reviewing is so common, so much dominates the discourse of book reviewing at the moment, that it's always surprising to me when someone suggests there are too many negative reviews around in the first place.
The advice often given to reviewers and critics who might have something actually critical to say about a book or a writer is to just say nothing instead, to let one's silence stand as sufficient commentary. In long-form criticism, this makes some sense; extended consideration of the flaws in a particular work or a writer's body of work is not usually as worthwhile a use of critical space as the explication of the most accomplished works and admired writers. (Although dissenting evaluations of established writers that question their critical standing certainly have their place.) However, contemporaneous book reviews serve a different purpose. The best reviews do not facilitate "consumer choice" or "cultural discussion" (although some reviews might inadvertently do these things) but take on the responsibility of providing an initial critical reaction to books that presumably seek an audience not just among those who happen to read this week's newspaper, and not just among readers this month or this year, but also future readers for whom the book's literary merit has become the most important consideration, not its purely occasional interest as a "new book" or as an artifact arising from its circumstances of its publication (the review's circumstances of publication as well). In some cases, those reviews provide the only available critical guidance to such readers, but even if additional, more extended criticism has subsequently appeared, often the presuppositions at work were introduced by the initial responses to the book or writer found in reviews.
This was certainly my experience as a fledgling scholar of contemporary literature, when I was trying to round up the critical responses to writers in whose work I had an interest. Of course, most contemporary fiction has yet to accumulate the kind of formal criticism associated with more canonical literature, but I would always look first at the reviews, and usually found they had in one way or another (in rendering both positive and negative judgments) established the context in which the writer's work continued to be discussed, the most thoughtful and conscientious reviews often enough still providing valuable insights into that work. Had I been surveying reviews motivated by the perceived need to reinforce a literary "community" against attack rather than to reinforce literature itself through honest and detailed literary criticism, even in the modest form of the book review, I would most likely have found little of value and concluded that such a "literary" culture doesn't really take literature very seriously.
Certainly no writer welcomes an unfavorable evaluation of his/her work. It's another thing altogether, however, to extend one's dissatisfaction with this sort of critical scrutiny to a general proscription of negative criticism in the name of fellow-feeling for one's putative colleagues. Not only does this reduce the creation of literary art to an exercise in group solidarity, but it's practically an invitation to writers encouraging them to do shoddy work. Since whatever I might write is going to be praised simply because I wrote it (lest I get discouraged), there's no particular reason for me to artistically challenge either myself or my potential readers. Even if I have mustered up some ambition and taken risks, if the result is met with the same bland acceptance as all the less adventurous books, what motivation do I have to maintain my ambition? And if it is met with the silence that is supposed to be an adequate sign of disapproval, what reason would I have to consider this as anything other than cowardly?