Willits is going to stay in the lineup even when GA comes back. If Anderson must play, perhaps it will be to DH for some games. Willits is a spark plug at the top of that lineup and Scioscia knows they would be sacrificing some offense taking him out of the leadoff spot.
It might be a situation similar to Ryan Theriot's where there is an odd day off here and there, but he is mostly a full time player. If you are carrying 4 OFs, a guy who plays 5 games out of 6 can still be hugely valuable.
On the other hand, if he stops hitting, and falls back to a 3-4 starts per week role, he should probably be dropped.
On the whole, if you need SBs, he's a must add right now at any rate.