There's a difference between having a conversation for the sake of clarity and understanding (good) and becoming argumentative/defensive (bad). There's also a third option, which is just saying "Thank you for pointing that out, it seems valid, but I'm not going to do that because blahblah"--which is neither here nor there, but is unnecessary.
As far as clarity goes--most words are approximations of what they mean. String them together, you get a lot of approximations conveying an inexact meaning. The idea is to get as close as you can to the meaning, but since different brains interpret things differently, you can't usually get more than "really really good approximation." "Cruel/harsh/thrashing/bitchslapping/punch in the tit/bullbaiting match in which you are the bull/gentle/nice/etc" are all approximations, and none are exquisitely clear, because none exist in an objective state. Most things are open to interpretation...
Blah blah there is no spoon.
I admire gentle critiques--those that turn tact into poetry and manage to call out a slew of mistakes without sounding like they're actually doing it. Those are really nice crits. (And, Unimportant, if your concern is being "gentle" enough, don't worry--your normal crits are pretty gentle in that regard
). I don't quite have that skill. I tend to come off naturally...sarcastic, I think. Or something. Even though that's not how I'm thinking of it...
But I try to make myself not sound like a bitch, and make it clear, in case people are unused to critique, that I'm trying to help and everything's all good and copacetic.
However, nobody does anyone any favors by withholding points of critique. Feelings pass, but whether they're good or bad, your manuscript may continue to suck if you don't do something. Part of the reason emotions are there is to incite you to do things. (...Probably all of the reason, actually.
I'm really good at science.) Thorough ("harsh," for some people) critiques can be very good for your manuscript. Which means they're good for you.
You know, like, how it seems that drilling holes in your teeth and filling it with a synthetic amalgam would be a bad thing...but...you gotta get the rotten shit out.