Thing is, you don't flash spam. That wastes MP like nobodies business at the early levels. One flash is enough for initial aggro retention then you Riot Blade combo bouncing it back and forth so that you maintain equal aggro on all mobs, you can flash again after a bit to solidify your aoe aggro but really once is enough if you know how to juggle your hate building sword combo. Any flash used after is in case of emergencies. Or if more mobs end up getting pulled that you intended.
Also, I've been in low level roulette many a time where the tanks just didn't understand how their skills worked. Gladiator doesn't play like most primary tanks in other MMOs do. (ie you can continually use your single target and aoe aggro taunts to keep aggro on you and that's really all you have to do in most other games) You can't just flash spam, not a with a good party. It doesn't grab as much aggro and if you are just using it and not doing your combos properly aggro is going to just get ripped right off of you by the dps or healer pretty quickly. And again, you'll run out of MP really fast.
I've seen many a bad beginning tank, thinking they need to spam flash, then they run out of mp and the healer rips hate off them because they are just healing like normal and they have no more mp for flash. Flash spam is just bad in general at the lower levels. I can't comment yet on how it works at the higher levels and endgame, but I'll find out easily enough after 2.4 (gonna level up my tanks more and take advantage of the mass ninja ques).
I did experience some good tanks as well, but I'd say the bad ones or mediocre ones outwieghed the good when I leveled up my Summoner, Black Mage, White Mage, Monk and Dragoon. I primarily did dungeons so I kind of know what I'm talking about, at least from my experience. I also have a real good friend who I leveled up my first job with (Summoner) who mains Paladin and I took mental notes on the things he was doing, plus he would explain stuff as we did dungeons together. I learned a lot about Gld/Pld tanking by watching him. He's definitely one of the best tanks I know.
So, when I started leveling up Gladiator and Marauder to the point where I could run dungeons on them players in my party that had obviously been with bad tanks before would be amazed that I'd keep aggro on 3-4/5 mobs at the same time without aggro coming off and another player being attacked (this was also with trigger happy archers). I'm not trying to toot my own horn, it's just this is what happened. It's what I've experienced. But I can't lie, it is a pretty nice feeling when you're congratulated for tanking well. There are many people that don't know how to play Gladiator and Marauder and tank well at the early levels. Hell there are people that are level 50 with i90+ gear and still can't tank worth a darn.