You didn't claim they were meaningless, but others have, and that part was directed at them.
As for your assertion, it's true that a player isn't going to play as hard for 36 minutes as he's going to play for 20, but first of all, that's besides the point if you want to find out how productive a player is while he's on the court. Whether or not he can stay on the court that long, the stats give you a good indication of what he does when he's on it. And also, players generally produce pretty similar numbers whether they play 26 or 36 minutes. Most players play a certain way instinctively and playing more minutes generally doesn't change that. For example, look at Marco Belinelli's stats...
http://www.basketball-reference.com/...belinma01.html
His per 36 minute stats have changed very little from last year to this year even though he's getting 10 more minutes a game this season.