If i may, i've made it to 36 with my "main class" and as you can (hopefully) see in my signiture i've been able to slowly raise my other classes up a bit here and there. But i hope as a person fairly new to this game (Bout 4 weeks now) perhaps i can helps some see from my perspective.
Let me preface:
I've played many many MMOs From Dragonrealms to GW1, EQ 1 and 2 Asherons call eve online etc, etc.
FF11 was not among them despite my lgeneral love for the FF series as a whole, my time zone and time available to play didnt allow me at that time to join in.

I had a very tough time when i started finding someone, anyone to join me in the Imperial Devices quest. A very nice person who has helped me again and again since brought me through it. He was level 50 all maxed it seems. I was very happy to finish the quest, however...

I did nearly nothing for it. I couldn't hardly land a hit before something keeled over which makes it less then fulfilling.

Then a few days ago i had put up in search that i'd run toto-rak if needed. A lvl 31(i think) person had been looking desperately (likely as i was) for someone to run his Imperial devices quest. It was fun and enjoyable. but it was much more fulfilling. We both needed to act to get through. we wiped on a boss (since we kept going) 3 times, and enjoyed it too.

The difference? Was really the trivialization of the content due to level. Not that i disagree at all if someone wants to have a lvl 50 friend just run them through. But for myself and the lvl 31 person, it was a much more enjoyable experience.

Another thing i often see, besides shouts for parties i can't qualify for, is a general lack of folks it many areas. For those that maybe saved a class incase a friend plays or are working on a unused class theres very few people, nevermind opportunities, to randomly group up.

My personal opinion definately tend toward level sync. It offers far more opportunities for everyone, both max leveled and new people like myself, to group and play and ejoy the content too. I see so much on the forums here about max level, end game end point. PL etc i sometimes wonder if theres much of a place for a new player like myself whom is so far behind so many. Many MMOs, by their very nature of levels and gear boosts and raiding etc make it tough for newer players to enter in. It sometimes feels like FF14 is even a bit harder.

Lvl sync can also spread out populations somewhat, since you can offer to group with nearly anyone, and still advance, it may be slower, it may not (often depends on how the system is set up and that is up to the Developers). But the options are much more open.

To claim simply that if sync gives the best exp/per X its a bad system is is to then infer that the PL system is also a bad system because it accomplishes the same thing. I personally have no issue with either system being the one players choose provided it is either sanctioned by the developers (or its use ignored and not punished) so this is a quick story of how i learned what Powerleveling in FF14 entailed

Concerning Sync and powerleveling. I'll admit i had no idea what PL entailed untill someone who i'd rather not name offered to help me, "level fast" I thought Party Leve questing or some such, maybe a dungeon run. Turned out it was this PL thing. After one mob which i ofcourse attempted to help fight against (and thus died) i thanked "someone" for trying to help and apolagized for wasting his time and left. I thought it was an exploit, and wanted no part. and even if its grey area, i much prefer leveling as i journey through a game as apposed to running to the endpoint. Granted its not that i don't move quick, things to be done, greenswraths to be quelled and all that.

The above gets to my point. I made a personal choice to not partake of this system. I would choose to use a level sync system. Not because its the quickest way, if it is thats also fine. But because i would be an active and needed part of that leveling group.

It shouldn't be a persons agenda to see that a particular choice be the "best way" of, in this case, level gain. And thus ensure that not only any other way, but any other system is either not put in place or beaten down to worthlessness to ensure that. As players my opinion is we should foster ideas that can be fun, perhaps not to you or me, for players to group, adventure, play and enjoy the game we all pay for.

(fyi i place opinion in many of my posts because they are exactly that.)

Thank you for reading another long one. Its a tendancy of mine