Ditto from me as from Will.
I personally never present my own negative subjective opinions on public forums (I do have them, but that's what mates are for) and I'm perplexed as to why anyone would ever want to. But that does not mean I don't think people should be allowed to voice those opinions, however critical or even begrudging.
What I do believe, as a matter of old fashioned manners if nothing else, is that no-one should ever voice an opinion in a public forum (knowing as they should do that their target is very likely to read it) unless they'd be willing to say exactly the same thing to that person's face (and I often get a distinct opinion this would not happen - which I think stinks to high heaven, frankly - so how's that for a subjective negative opinion?)!
So when the nibbling looks like heading towards a feeding frenzy I often offer positive counterbalancing views in support of fellow artists, (or just other human beings) - because I know what it's like up there (and getting there). Trust me it can hurt a lot more than you may realise - and I know people who've given up just because of unreasonable criticism which came at a bad time.
This is not really why I've raised objections in this thread, though.
What I personally object to, and usually speak out against whenever I encounter it, is unfair or inappropriate criticism (such as suggestions around selling out, underhand dealing, favouritism, abuse of The Tradition / public funds /airtime etc), when that opinion is presented as objective when it is in fact highly subjective (and often colossally ill-informed).
Not liking an artist or performance and saying so is one thing (though I'd rather people said it in private or to the artist in a tactful, positive way). Suggesting they should not be an artist at all - or be performing such and such in this or that arena is something else entirely.