What reason do they give for suggesting dropping the keel?

    Is it a cast iron or a lead keel?

    If it is lead, then the rust staining is probably indicating a problem with the keel bolts, in which case dropping the keel may be a wise precaution. After the bolts are checked, the hull to keel joint can be properly sealed so the problem does not reoccur.

    If it is a cast iron keel then a bit of rust staining at the keel joint is quite common. Maybe all that is required in that case is a bit of sealant around the joint?