If everyone could upgrade or control the sentries, it would instantly be ruined, because absolutely everyone has their own sentry, and the server's normally for 20+ players. (extra lag common). Welders would make the choke point so choked up you couldn't pass a bullet through it. Also, Medkits and grenades are already in it, in dispensers are located within the bases. As for the sentries, they were intended to be just temporary installments for defense, forcing the teams for fight even harder.
I may be hosting on the weekend or on Thursday, see if you can come, since you evidently haven't played it yet.