The Comptroller General ruled that "A firm's failure to attend a site visit is not a valid basis to reject an otherwise acceptable offer" (see p. 5 at this link to the decision). In light of that ruling, it may not be the best course of action to require prospective offerors to attend a site visit. I don't know of any reason not to offer a site visit (essentially a pre-proposal conference) so prospective offerors can be better informed on the requirement. However, the Government's requirements package should ideally be sufficiently detailed and forthcoming so that accurate proposals can be prepared without a site visit.