I disagree, actually. It's better for the rest of us if the engie CAN move. Why? It means he has to make additional judgements (IE making it a more challenging, worthwhile weapon); he needs to guess if the sentry can fire at what he sees; probably he'd need to stay in sentry range; he needs to not run into the firing line if necessary. Lastly, he needs to know what's behind him (and if he turns around, he's prolly aiming the sentry too). Slower sentry turn speed than default engie means he's already defenseless if he turns quickly and DOES see a spy; the spy can just shoot him if the sentry is pointed opposite of him, as his reaction time will likely be too long. Then, just blow the damn thing up.
Besides, engie will NEVER (aside from that initial jump) really catch on at ChHa; we've got a hell of a spy population =P