If Ukraine is a member of NATO, then we have to go to war against Russia if they invade. You’re willing to do that? All for “sticking it to the Russians”?
If Ukraine is in the process of obtaining NATO membership it may go a long way to getting the Russians to step back from a full invasion. If they do invade and annex all of Ukraine that puts Russian forces on a very tense Polish border. The Poles are not going to be the least bit receptive to having Ivan at their border again.
Because it worked out so well the last two times.