Well, I already have a line in my functions that defaults to this. However, the attachment page still exists. The public can still get to it. And if I go to my list of media, and click the 'view' link, it will always take me to this attachments template.
I'm guessing the idea of having this attachments template system came with a very early version of WordPress, and it's just sort of outlived its usefulness. Maybe I"m wrong, and people use it a lot. I don't know. All I know is that I never use it, I don't want to take the time to go into my themes and create templates for attachments, and I just want to obliterate them from my installation.
There's a plugin called 'remove comments absolutely' that performs a similar function for people who don't ever use the commenting system in wordpress. It removes every trace from the front end and the admin. I was hoping there would be something similar for attachments.