There are appropriate uses of the level sync function, those uses include things such as ARR content being synced to level 50, HW to 60 and SB to 70. Also level sync for quests is acceptable.
Inappropriate uses are essentially everything else that is done in game, let me explain. Our entire goal in this game is progress and find better gear, which means we should be becoming stronger, as we become stronger older content should become easier. But that is not how the game plays out. We do get stronger over the course of a patch, but then we are forced to become weaker for older content while at the same time older contents rewards are being diminished.
Example! The Expert Dungeons. When the expert dungeons are removed from the expert pool the amount of current tomestone they give us is reduced, and we are greeted with a forced drop in item level. Which means, the dungeons never become easier, but the reward we get from it continues to be reduce with each patch. I see this as counterproductive, why get stronger, just to be forced to become weaker? The devs could easily just reduce the rewards the dungeons give but allow us to retain our current item levels.
The newest example, and the main reason for this post is Eureka. You have added a way for people to "solo" the content in the future by allowing us to purchase Pazuzu feathers. But at the same time, we are reduced to level 20. In essence this allows people to form small parties and kill the lower level NM's for farming, ignoring all the higher-level NM's because they will be too difficult for the low number of players who will ever find themselves in Eureka Anemos at a time.
What would seem more sense in my mind, is to not level sync us while entering Anemos, allowing us to enter at 35. I'm sure a group of level 35 WoL could reasonably fight the high-level NM's in Anemos. Doing this would allow players to actually form small parties to farm various gear instead of relying solely on farming lower tier NM's.
Essentially, continuing to lock content being level syncs makes us feel like we're not actually getting any stronger as players, especially when we go back to farm some of the older content. It also makes things like Eureka and Diadem impossible to go back to even with small parties. Content that is patches old should be easily handled with parties much smaller than originally required.