If a sentry even gets one kill, it's done its job. If a sentry is just there to divert the enemy's attention, it's done its job. A good engineer doesn't care how often he has to build, he just knows what, when, where, and why he has to build. And he knows when to build and when to own face with an actual weapon.
Conversely, if there's high teamplay, as you claim, an engineer's team will help protect their defenses. ;P