It's lighter and easier to deploy to the theater. Once there however, it does indeed have the problems of no protection for the redlegs and lesser mobility than self propelled. It's also cheaper, and in these days of low defense budgets, that is important too, much as we'd like it to be less so. We also have lots of it still around and it still shoots.
While it may not be artillery, let alone towed artillery, something is whooping up on Baghdad as I post this and as the sky lightens towards dawn there. Oops, that last set sounded like outgoing tube artillery. So for our forces, or the Shiites in other parts of Baghdad, "INCOMING!".