Yeah, I see what you're saying. The idea of keeping points throughout the season based on how your team performs on each particular game sounds like a good idea...however I still see two problems with this--
A) If I were in a league, and I could gain easy points by doing 1v1 matches, I would be trying to do as many as possible. Now try to fit that in an open play day with 30 people requesting that between every game...
B) The purpose of a walk-on-play day is generally just to have fun, shoot your friends, etc etc.. Turning the average just-for-fun walk-on play day into a competition every weekend sounds like a bad idea to me. (But that's just my opinion) I can compare this to when I used to race Karts in a racing league at M.I.K, Normal days were all for fun, if someone got wrecked they just laughed about it and put it on Youtube. But when raceday came around on Sunday and someone got wrecked, there was shoving and punching, pissed off parents, and people throwing helmets. Not cool. That's way more extreme than anything I've ever seen at Field51 but still, competition creates a more hostile environment and isn't exactly my idea of a walk-on-play day.
Some examples of this would be:
Excluding new players so your team wins, so you get your points.
It definitely won't help the overshooting problem that some players are complaining about (See BPS cap thread)
And of course, unfortunately, A motive for people to wipe. (For those that have the little devil on the shoulder saying "WIPE WIPE!"