I think the Imperial phase is simply the natural prosecution of a "winning" culture. An Empire forged by sheer military power will collapse instantly, but a lasting Empire must have some culture behind. For example, the Islamic Empire spread from Morocco to India and had a lasting legacy cause it had a well-formed culture behind it. The same can be said about Alexander's empire. Rome was different (and in my opinion better) cause its culture grew and grew with every conquest. Romans were intelligent conquerors, they knew how to exploit both the riches and the knowledge of the people they vanquished.

And, well, one can be anti-imperialistic as much as he wants, but (and I'm only giving what I consider the finest example) if it wasn't for Romans who conquered Greece we would probably not have heard of ancient Greek culture at all :) Sadly, the lasting of a culture requires military power and political stability, that's almost a universal fact in history