The question is why.
1) It doesn't hurt
2) If we only ahd Hill only (but not Smith) and the Hill got injured, we'd have Huard as starter and Davis as back up to carry us through the season. How far could they carry the team?
3) We don't want to risk putting Davis on the practice squad and another team claiming him
4) Singletary basically gave the Job to Hill, because there was no separation between Smith and Hill (No confidence in either QB at this point). Because of this uncertainty, we need more options at QB during the season in case both Smith and Hill are ineffective.