I'm gonna have to go with "Against Level-Sync" for now. As I can see it being useful in the future, with the balancing of things and Battle System changes in the process right now, it's not a good idea till things get sorted out. Seems like some people think you just flip a light switch and then we have Level-Sync. There are way to many things to look into to add it right now. Examples:
R50 Armor/Weapons - Stats Level Sync to R30? for example
R50 Skills - Way more effective at Optimal Rank than @ Lower than Optimal
Due to the balancing issues SE would have to work on ATM to add Level-Sync, I feel their time is better spent working on other "Content" to add Fun to the Game, rather than an easier way to level.
Because nothing is stopping anyone in this game from playing with their friends ATM, it only hinders the ability to level up effectively with "Friends" ATM whom are at a lower/higher Rank. There are other more important issue than this currently.