I'm a full i90 with Allagan Scepter BLM. My AA does about 11 DPS if there are absolutely no interrupts, and much, much less if I'm actually casting spells - I peg it at slightly less than 1 out of 2 spells I get an AA off, including Swiftcast and Firestarter procs. Thus, about 5 DPS increased. Since Scathe is a hilariously bad DPS loss and has no place in the rotation outside of its utility for moving and instant damage, there's not much justification there.

It's hard to see the benefits of AAs for BLM in everyday use. Even one GCD lost due to movement you wouldn't have been forced to do negates anywhere from a bit more than a minute of AAs up to three minutes+ depending on if you hit a crit or not. A lot of enemies have tail swipes which would force a move, as well as the negative of sharing a spot with melee, which increases your chance of having an AoE target you compared to having free reign over the field to spread out. Not to mention, it would take a proc or two to get in range without losing GCD time.

However, it is good advice even for BLMs in certain times. It can certainly have its benefits if you care to move after a Firestarter and the enemy doesn't have anything to impact your rotation by staying behind. Turn 5 is a good example as mentioned; for phases 1 and 2, there's virtually no detriment to clicking Twintania while stacking in your groups. Likewise for the snakes, if you can move in for free. The AK boss, WP boss, Ifrit EM, and Titan EM have you stacking for large periods of time on their rears without any negatives, and clicking them for a roughly 5 DPS increase is quite worth the effort. On the other side of the fence, T1-4, and a large portion of bosses and trash have AoEs and cleaves and mechanics where going for that 5 DPS increase just will in all likelihood lose you a GCD and completely negate their benefits.