This is a really good point. The idea is attractive, and I'm not overly against it. I mean, the ability to sync down low exists now, and yet you'll be hard pressed to find a well rounded group of Lv. 60 characters who will sync down to a Lv. 20, because of the loss of abilities and skill ups. SMN burns are always going to be a stain, because honestly, how can you prevent them?if someone did that then we would have summoner burns all over the place.
Level Sync is probably at the optimal level right now, and it was a bold move for the game when it was introduced. It allows for more opportunities, but has just enough in the way to keep it from being broken. People have constantly been asking for above sync-cap skilling up to be possible through syncing, but this would give almost no incentive to ever party in higher level areas. To me, doing that would carry a larger consequence than allowing level adjustment. But either way, level sync should be left alone for now. If partying outside of Abyssea for lower levels becomes a tremendous issue again, then possible modifications can be made.