Let's just break this down to a situation that us non-millionaires can all relate to.
Paul Pierce goes to work at 7pm on Wednesday night. He's playing poker at 9:30pm the night before. 20+ hours before he has to go to work.
This is like the equivalent of a normal person having to go to work or school at 8am on Monday for a major presentation or test, and choosing to play video games at 10am on Sunday.
Does that really sound like a big deal? Especially if you've been preparing all week or year for this presentation/test? Pierce has essentially been preparing over 30 years for this, and all of the last 5-6 months for this season in particular, and all of last week for Philly specifically. If he chooses to relax a little bit the day before the game, that's really quite all right. Actually, don't most "experts" even suggest that it's best to relax the day before an important test instead of staying up all night cramming? Sounds like what Pierce is doing to me.