Enterprise was used for a lot more testing than just the landings. It was put through a lot of vibration tests up to and including the "boilerplate" configuration (orbit on external tank mounted to SRBs) to stimulate the stresses the the entire vehicle would undergo during a launch. Enterprise was supposed to be fully outfitted as the second space shuttle after completion of the tests, but there were enough design changes between it and Columbia (and presumably the rest of the orbiters) that it wasn't cost effective. NASA considered this again after the loss of the Challenger, but they basically built Endeavour out of spare parts instead.

Enterprise did her job even if she never made it to orbit. It just want practical to do the work needed to make her spaceworthy after the design changes from tests performed on her (presuming the design changes that made it impractical weren't because of the USAF's need to launch from Vandenberg).