Side quests, especially 51+, are in a terrible spot of being generally useless.
The only ones that are worth doing are:
Plus quests - they unlock something
Beast tribes - they give massive experience
A lot of the 50 quests don't give experience but they are either Plus quests, or they at least have some kind of achievement somewhere along the line. So they still have some worth because there is no other way to get those achievements (and any of the reward they gives out).
The regular 51+ side quests however are almost useless. They don't really go towards any achievements, as even the ones that are part of a massive quest chain don't seem to have a unique achievement for finishing said quest chain. The only achievement they could possibly go for is the one to do unique quests, but since it only requires 100 of them, anyone who does MSQ will complete it long before they reach the HW/SB areas (ARR alone has well over 100 MSQ to finish that achievement with).
This would be ok if they gave really good exp, but they do not. I just did a level 55 side quest on a level appropriate secondary job (55 BLM), and it gave me 33,600 experience (out of 2,217,600). That's 1.5%. There are only 34 level 55 side quests (one of which is a DOH quest), so that's only half a level if I did ALL of them, which would still take a few hours because a lot of them just make you fly back and forth across the map.
Considering that regular side quests are once-and-done content (you can never repeat them, not via allowance, not via daily, not via queuing) there should be SOME incentive that makes them worth doing other than just completing for the sake of completing. Bad exp is not enough because it means I could spend that time doing something else and not have any trade offs at all.
I could do leves, fates, potd, dungeons, pvp, pretty much anything except for pure monster grinding and in that time get more item/currency rewards, more exp, and probably getting some credit towards other achievements.
Even expanding the Storyteller achievements to go to 500, 750 and 1000 with some titles/rewards (and then gradually increase them as new patches/expansions come out), essentially keeping the storyteller achievement to be all MSQ + at least 80% of all existing side quests, would still give some reason to do them other than just "because they're there so why not".