If you had 1.3 installed on a server farm, running PHP, mysql, who knows what else, everything getting along good, and you had it patched and secure, would you risk breaking a few thousand websites to "upgrade" to 2.0?
2.0 is working well, the bugs are out of it as far as I can tell, but it is just not worth messing with if 1.3.27 is working for you. I only install 2.05 on new installs myself, but would be hesitant to upgrade a busy server from 1.3.27 for just the sake of upgrading.