In a social/friendly context, something feels weird about withholding full agreement with a plan. That is, say someone says "We should go get dinner and then go see a movie" and I say "Yeah, let's get dinner." That is: I agree with some part of what was proposed, not directly addressing the rest. This feels off, but I'm not sure why.
I think it's the subtext of being unwilling to disagree explicitly. That could be something about them, something about me, or just an awkward reminder of the degree of uncertainty and distance between us — that we don't yet know if we can disagree well.
A different approach to the disagreement might be to say: "Yeah, dinner sounds good but I'm not feeling like a movie tonight." That is, I could explicitly point to the disagreement. That feels somehow more honest, less withdrawn or manipulative.
And yet — sometimes the "go see a movie" is not so explicit yet that I can even tell what I think. It is too vague a possibility yet to have strong feelings one way or another. And I am feeling lazy and just want to comment on part of it rather than sort out my feelings about whatever (else) might be there. I'm mainly thinking here of contexts where no decision needs to be made immediately — so that not sorting out what I think costs little... except a piece of my sanity.