Tony Allen isn't enough to make our team defense much better, and would hurt our offense as much as he helped our defense.
A better option than Tony Allen would be Gerald Henderson. He's a solid defender, good offensively with a more than adequate 3 point shot and he is familiar with the players and the system.
If we had just unloaded Evan Turner instead of Allen Crabbe, then sure. But it doesn't matter what we think: the unloading of Crabbe was a money-saving measure, so it doesn't seem likely that it's the precursor to more spending.