If one does that and by doing so one changes the spirit of the original, then that is intellectually dishonest.
However, often one is merely addressing an point within the original quote (often its just an question of a single fact, for example) that can stand alone.
In which case, it is a waste of the readers' time to post the entire quote when the objection (or comment) is addressing only that small data point.
I'd agree with that, but at the same time, I hate to strip the thinking around the thought out. I like the part responded to to be highlighted and the surrounding thoughts to be included.
Many times, the writer leads up to a thought with others and stripping them away lessens the intent of the writer.
Yeah, understood.
There are times when every sentence in the original is so dependent on every other, that parcing out any single sentence to comment upon is violating the spirit of the original.
OTOH, when the original is based on an assumption or fact that is debatable?
Then parcing out
that fact to study and comment is probably a good idea.
And let us also acknowledge that reasonable people can still disagree about when it is fair to parce out a fact, and when it is not.
Often what some of us see as intellectual dishonesty is an honest disagreement about what is germane.
These are, of course, judgement calls we need to make in every specific case.