I've blogged before about how critiquing works best if there's a good fit between critiquer and critiquee, but it still amazes me how many aspiring writers think that positive comments - even those on their own work - are useless.
The praise on the outside of a praise sandwich is far more than mere sugar to make the filling more palatable - though side-stepping natural, human defensive deafness or resistance is one of its functions. Yes, a vague "this is wonderful" is no more useful a critique than a vague "this is shit", though it hurts less. And yes, if you've got 20 sums right, and 5 wrong and so only got a B+, concentrating on what's wrong with the wrong ones is probably the way to an A- next time. Mind you, it's concentrating on how many you got right - holding on to the fact that you're worth it - which is most likely to give you the energy that it's worth putting in the work.
But there's something far more important at stake. No creative worker ever knows quite how something will come out, or why it comes out how it comes out. And, by definition, none of us can know how our work will seem to someone else. So discussing what does work in someone's piece is just as valuable, much of the time, as discussing what doesn't work: a mirror should show you both how gorgeous your dress is (no need to change it) and the thread hanging down from the hem ("Mu-um!"). Without that mirror, the artist may genuinely not see either thing. Positive critques aren't "blowing smoke up your skirt" or "bullshit" or whatever pejorative term you fancy: they're feedback about the reader's experience of your work.
Why wouldn't you want feedback on what works and ideas as to why, so you can do it again? Working on a piece of writing is just as much (actually, far, far more) about trying to affect the reader in the way you want to affect them, as it is about trying not affect the reader in wrong way. So a feeder-back who only tells you negative things - even if those things are quite useful - is only telling you half the story. So why do they do it?
1) You've asked people not to "waste time" on telling you what works. Yes, lots of fluffy comments about how lovely it is don't get you much further, and yes, when I've had a rejection, being told "But they said it's really well written" doesn't comfort me much because of course it's well written: that's my job! But reading practical, specific details of what works (as well as what doesn't) for that reader in your writing is never a waste of time.
2) They genuinely can only identify and explain negative aspects of your work - which probably means anyone's work including their own. This is all to do with how they're wired (parents and teachers have a lot to answer for), but still, they're only 50% useful, as opposed to the 100% useful that other readers might be. By all means make use of what they say, but (as when listening to anyone's story of their break-up or sacking) remember that it's always only half the story.
3) They're being deliberately nasty in withholding positive things that they could usefully say, but choose not to. In which case, their motives for giving feedback are clearly selfish, or actively malicious and they're not a person you want in your life - specially not your writing life.
4) They have a macho delusion - again, probably about themselves as much as others - that positive things are fluffy nonsense, and if there's no pain, there's no gain.
But can any of us actually swear that we know precisely what we do well and how well we do it? The more developed a writer is, the more they're aware that they can't read their writing as others do, and only with feedback can they be sure they're working on the reader as they wish to. So one could argue - I am arguing - that it's a form of arrogance in itself, to say that precise, positive feedback is useless: such writers are so sure that they know their strengths that they don't need them pointed out. But they're wrong, because no writer can know all their strengths.
And what's more, all teachers know that the weakest students of all are the ones who haven't yet learnt that they're bad. So, if that pain in the bum on your favourite forum is busy declaring that they don't need positive feedback, or busy declaring, as they put the boot in, that you're fluffy and un-serious for objecting to their offensive comments, you can safely assume that a) they don't know how writing works and b) they don't know how people work. In which case, their feedback isn't worth two seconds of your time.