I'm honestly shocked it's taken people this long to figure this out. Factor in the waiting time of FATE's - FATE's can be random in when they choose to pop - this is time spent idling for most people.. Also factor in travel time, a lot of FATE's require a good bit of travelling to get to even while mounted. Add that time up, along with the time it takes to complete the FATE and see what your reward is.. It's actually not that great. lol
If you spent all of the same amount of time you do on FATE's, you'll make at least the same amount of experience, if not usually much much more. I've done this before with people when leveling my main, and we were just RAKING in the experience points.. Mind you, if a FATE did spawn near us, we'd do it, but if it was far off we wouldn't bother with it. Really this whole FATE zerging obsession people have is based off one thing and one thing only.. instant and easy gratification. People go in, do not work terribly hard, and see a big pretty number. People figured this was exploitable and decided to start spamming them, end of story. Fact is though, FATE's are not the best method to leveling.. Old fashioned grinding is, and it always will be. The only downside to grinding is you don't get seals, and you're making less money - you might also be suffering some gear damage too in an efficient party.. But eh, if we're talking about purely leveling classes, then those things don't really matter so much.
Also as for the naysayers mentioning mob respawns being inconsistent.. actually it's quite the opposite. Mobs all have relatively short respawn timers, remarkably fast even for an MMO in some cases. Mobs are also on very exact respawn timers.. FATE's on the other hand do not appear to be exact as to when they appear, and there seem to be a lot of factors we don't currently understand (and may never understand) as to what causes them to appear. I've honestly spent several solid minutes idling in town while waiting on FATE's before, whereas other times they're up back to back to back in different areas of the map. You're going to have a much easier time relying on mob repops than FATE repops.