Agreed... Unfortunately, there isn't a great way of knowing who's going to have the better game before the game. So what happens is coaches typically lean towards the player who has performed better previously in games/practices. For example, Grant has been better throughout his career, better this season, and even was better last game vs Golden St. Just because a player has a better first half, doesn't mean he's more likely to have a better second half.
What we did see is Billups played Grant 7 minutes less than his season average and Walker 5 minutes more. All things considered, I think Billups has been more flexible with minutes than many of the great coaches would be.
Grant is better than Walker, it's not even close. Be thankful we suck so badly that guys like Walker are given a longer leash when they do play well because stuff like that won't happen near as much if we ever get good again. Not to mention, if we do get good again, a lot of the young guys you love will be the vets, taking minutes from the new young guys you'll love, while you trash the vets that you used to love.