Anytime you can have extra time in a park go for it. I found the Halloween fireworks to be the best I've seen at WDW. The later you stay the better the party becomes. Crowds thin out after the fireworks. If you skip the parade and fireworks, you will love how fast it is to get on the rides, etc.
Disney controls attendance for this event. This means it doesn't get as crowded as it could get at times. My one hint would be to go to a different park during the day and use the Halloween party to see MK. Then you'll have another day to enjoy another park rather than MK; because, you were able to see it at night. I have to tell you some rides at night are fabulous like Thunder Mountain. If you really like MK then you can have an extra day at MK. It's a win win either way.
You are there anyway; so, I believe in utilizing as many hours at the parks, as possible. Now, if you can't handle a long day then you may not get your monies worth. Noone can speak for someone's stamina.