Considering I usually don't have the ability to wait the duration to get a group and then stay in it for a long enough time to gain levels that fast, Your argument doesn't work well for casual players.

(Same with my friends, who have yet to even reach Lv25.)

Again, It's kinda hard to PL them when I am only Lv34 LNC.
It just makes for boring content w/o any challenge, that isn't quick enough to get them caught up.

Your ONLY reason for "No level sync" is... "Because I said so!"

or because "It will be taken advantage of!"
yeah... because a Lv50 CNJ PLing a group is not currently breaking the leveling system?

The ONLY argument that can LOGICALLY go against Lv sync, is the time/resources it would take from the Devs, that could be spent in other places.