I don't think an all-rounder has to be equally good at batting and bowling, or selectable on either alone. An all-rounder is a player who is selected for a side based on BOTH his batting and bowling. A bowler who bats a bit (like say Vettori) is not an all-rounder, and neither is Gillespie. The likes of Pathan, Boje, Warne, Wasim, Giles etc from recent times also fall into this group. Most all-rounders are noticably better at one discipline than the other, but the fact that they are considered acceptable practitioners of both is what makes them an all-rounder.
Sobers, Watson, Kallis etc are examples of batting all-rounders, while Pollock, Miller, Hadlee, Imran Khan etc are bowling all-rounders. Only a select few all-rounders like Kapil Dev are genuinely about equal at both batting and bowling, and even fewer (Miller, Sobers and Imran are the only ones that spring to mind) could actually make a test side on either discipline alone.