Primarily to keep consistent with Merlin when I backported the disable running of scripts update. Secondarily, my sense was the most users make use of jffs and it was one less thing to worry about. But, I'm open to differing opinions.....
OpenVPN key/certs are now stored there, and expect Asus to move more content there, as they try to get around the 64 KB nvram limitation of current models. Since there are zero reason to NOT enable the jffs partition, it makes sense for it to be enabled by default.