One off-on the spot, especially uncalled for criticism is not particularly helpful in my experience as it immediately puts up a defence mechanism - a singer is unlikely to take it into consideration in these circumstances.
On the other hand, if a singer asks for help, I think he or she should be prepared for comments - adverse or otherwise, on what he or she is doing, and more often than not is prepared to accept such comments in the spirit they are offered.
I cringed dozens of times when I heard singers approach MacColl at clubs, having just sung, and asking him for comments, when what they really wanted was for him to say that it was the greatest thing since sliced bread.
He invariably gave advice, sometimes detailed and I've overheard extremely resentful comments later - it became part of the myth that MacColl "told people how to sing".
I think it was ill-judged on his part, whatever his reasons for doing so - quick-fixes like that seldom work.
All criticisms should be balanced in a way that a singer can build on their strengths and reconsider their weaknesses - stick and carrot (presuming the singer agrees with the criticism, of course - in the end, it is they who are singing the song and it is their choice what they do with it)
Jim Carroll