You shouldn't have to pay transaction fees for rebalancing in your 401(k). If you do, it's kind of a rip off.
That said, monthly rebalancing is almost certainly overkill. I just did monthly because I was working with a monthly series of data, and was assuming monthly additions to the portfolio.
There have been some studies (it's not difficult, but it does take a heap of data, especially if you want to go back a good distance and use longer periods) of the differences in long-term returns which come from different rebalancing frequencies. Here's a pretty nice one:
That's precisely what it does for you. It's a way to systematically buy low and sell high - in addition to keeping you on track for your target allocation which would, over time, drift if you didn't. And the drift without rebalancing is exactly in the direction you likely don't want - it's towards the assets which have had the highest returns (and likely present the higher risk).