Tradition in my family is to spend our last night of vacation at Magic Kingdom. I'm looking at the calendar from last year and I realize that that may be on a night where MVMCP takes place (so there is no Wishes). Is it worth it to buy extra tickets to go to this event or not really?

The question of "worth it" is something only you can answer.
It's no longer "worth it" to me, but then I was at some of the early MVMCPs where I feel there was a lot more value for a lot less money, and smaller crowds.
We went last December. I can honestly say that I was disappointed in the crowd level. I think they now over-sell these parties. We went to a Pirate and Princess Party a few years ago (when they still had them) and the crowds were way smaller for that. The Christmas party was lower crowds than during the day, but certainly more than what we expected. We went mostly for the rides, but they certainly weren't walk-ons. I doubt we would go again...maybe the Halloween party next time.
We actually left early (right after the fireworks) because we were all half sick and just wanted to go home and sleep rather than ride the rides over and over.We went to our first MVMCP in Nov. 2013 and had fun but it isn't nearly as fun as MNSSHP. The lines weren't too bad and the shows aren't too bad but it just didn't have that special feel like MNSSHP did for us.