OK, so there's Tapscott's explanation: Players have to earn their time on the court. If they lack experience and get caught too often out of position, then they're gonna get limited playing time.
Then there's cuppettcj's explanation (hope I get this right, sorry if I don't): Grunfeld's planning on trading McGee who's trade value is based on his incredible upside and not on anything he does on the court this season, so it's better to keep him healthy on the bench.
Now I personally think the Wizards want to keep McGee because they should want to keep him. Trading him would be a big mistake unless a can't-miss-blockbuster trade comes around, and I just doubt anything quite that good will be available. The Wizards have enough young talent or solid veterans to trade along with a lottery pick so if Grunfeld wants to make a big splash than I don't think he needs to trade a player who could in the long run be the one that finally makes this team a contender.
So finally another theory (this probably isn't original): McGee is going to be a reserve behind Haywood again next year. Limiting his playing time this year means there's no controversy next year. It also means that if McGee's development is more gradual but steady rather than rapid, yet inconsistent, then when it comes time to sign McGee to another contract perhaps the Wizards can get him for something a little more modest? Certainly his statistics wouldn't be quite as impressive, but the Wizards might think in the long run McGee's development might benefit just as much.