Ond_kajaThe tanking ability with the rescue ranger is pretty poor compared to the wrench. Definitely won't tank very much before the engineer is forced to pick it up, at which point you might as well switch to another class if they are pushing because you are useless as an engineer if you don't have your sentry up, especially if you don't have the shotgun. It does provide more options but it's not really that OP.
but repairing with a wrench puts you at risk for being killed along with your sentry and it takes a few seconds to run back to spawn and switch classes, now you can repair the sentry from literally inside spawn with zero risk and then switch the instant it goes down
plus there would also be certain situations where you could repair with wrench, run to spawn and then also repair from spawn to keep it up a bit longer before switching classes