Here's the thing--I think what you're seeing is a mix of multiple objectives going on at the same time, thereby making the team strategy a bit complex and nuanced:
1) The Jazz want to be "playoff competitive," to put a decent product on the floor, keep the Jazz winning tradition alive, and give the fans something to cheer about. Super-smart fans might advocate for a clean rebuild, but there's never a guarantee it will work, and it's a risky strategy for the casual fan that buys seats and hot dogs. If the team is good enough to be in the playoffs, it's hard for the front office to make a U-turn and not make the playoffs. Yes, the Jazz could trade Al and they probably came close to doing so. However, he's been about as effective on offense as Patrick Ewing was during the latter years of his career. It's hard to give up that bird in the hand in the middle of a successful playoff run.
2) The Jazz want to improve the team and move up to the next level. Generally getting lottery talent or top FAs (which we generally suck at) is the way to go. So as a matter of policy, the Jazz have to draft well and they have to trade well. The FAs they are able to attract are generally wayward vets who are looking to reinvent themselves or just plain stay in the league. The Jazz have to play their trade cards well and try to pick up extra high-value picks in trades.
3) In order to achieve 2) above, the Jazz have to protect the value of their existing players and "sell high" in trades. They moved Deron when his value was at its peak and when the market (i.e., Nets) was most desperate. (This was a good move....especially when you consider paying DWill $100 million is a bad move. He couldn't carry an expensive team past the 2nd round.). So in order to protect the trade value of Al, Sap, Foye, Marvin, and even Earl Watson, these guys have to play significant minutes. And as long as these guys don't suck seaweed, it's generally for the best.
4) The Jazz need to develop the young players they already have by making sure they get good minutes every reasonable opportunity. However, since Corbin is trying to win games (Objective 1), he's been giving minutes to the vets, simply because he trusts them in late-game situations more and he wants them to be in a rhythm. Burks should be getting more minutes at the expense of Marvin, Earl and Foye. Kanter should be getting more minutes at the expense of Sap and Al. Hopefully after the trade deadline this will happen.
