I don't think I've ever heard a compelling argument for not doing this. I know some say it is a security issue , but why?

If your ATA is after the DSL Modem (mine is a RT31P2) and is feeding your main router (mine is a WRT54GL) isn't the router still handling traffic flow in and out or your network?

DSL Modem-->RT31P2-->WRT54GL.

Thanks
Jeff