Tony Vargas
Legend
On the one hand, it's not anything to worry about, since perfect balance, let alone precise equality is impossible, on the other, that's the hypothetical (or close to that ideal, perhaps) the OP is asking about.All things being balanced and equal just isn't fun. Further, you really can't optimize anything under those circumstances.
But, even the specter of idealized balance can't stop optimization, it just doesn't reward it, at least, not in the same way. You can still optimize a build in a balanced system, you're just optimizing it to a concept or purpose, rather than flogging the last fractional point of DPR (or whatever you're trying to push higher than the next guy) out of it. The point becomes differentiation and optimal modeling, rather than out-performance.
"Mechanically equivalent" and "no difference on the effectiveness" could be interpreted very differently. The former could be read as "no meaningful difference," which is the antithesis of balance, IMHO, while the latter could be read as 'balanced,' assuming there are still meaningful decisions to be made...That doesn't change, what changes is that both characters would be mostly mechanically equivalent.
So back to the real question of the thread. Would you want to play in a system were optimizing had little to no difference on the effectiveness of the character?