Bypassing the stat cap is really a crappy reason considering that we have total control over the CAP and can make the BST whatever we want it to be. It would amount fundamentally to a disguised flavor argument. The real use for Pure Power is in doubles, allowing the pokemon carrying the ability to swap it to a partner.As somebody mentioned, Pure Power allows us to bypass the stat cap that is in place. Even with a base 30 attack, it has the ability to reach base 110 attack, which gives a lot of leeway for the other stats as it barely affects BST. It's hard to undershoot something like that.
Unfortunately I don't think doubles are a concern for CAP. They certainly haven't so in the past.