I think this conundrum can be broken up by noticing how pervasive one extension is. A feature that has become identical with the core product in the eyes of a layman is problematic in the same way as thinking "IE6" is the same as "the web". If there's no awreness of choice, then the only outcome is further lock-in.

Implementing "sync" on its own doesn't matter that much, because few people think "sync" is a defining feature of the web standards, or that preview is an inherent part of web feeds. The awareness of alternatives still exists (I hope).