There are plenty of teams that I could think of that use CB Scizor as a sweeper, CB Tyranitar as a sweeper, and even Specs Pokemon such as Latias. It's easy to build a team around a Choiced Pokemon, especially around one such as Aggron who only needs to use Head Smash to defeat most Pokemon.
A few pages back, Justinawe (I think) posted a long post with Pokemon in OU who were OHKOd, 2HKOd and 3HKOd (or farther), and we all saw that it 1-2HKOd most Pokemon. This means it doesn't need a precious set-up turn to sweep, and you don't lose the precious power you gain from the Band itself.
As for your sentence, "...you need to think of when it will come into play...," think about when CB will be coming into play. You aren't sending it in early game, but rather mid to late game, where most Pokemon are at <3/4 health. As we saw in Justin's post, Aggron gets a ton of near OHKOs on Pokemon, and if they have taken prior damage they are OHKOd. As for RP Aggron, it's loss in power is keeping those Pokemon at 2HKOs until they reach signifigantly lower HP, meaning you're basically forced to keep Aggron until the end phase of the game.
Recap: Plenty of Choice Pokemon pull off sweeps, you probably just don't ever think about it being a sweep. CB Aggron can come in earlier then RP Aggron and provide the same, if not better, results. The loss of power in the RP set means that you won't be KOing Pokemon as often, and you have to hold it until the end phase. I place emphasis on the Plenty of Choice Pokemon can pull off sweeps, though.