Speaking of AST, here's a real life example of how haste was bad for BLM in the past when AST could haste:
Once upon a time, BLM MP restoration during ice phase was per 3s tick rather than per spell. When AST was first introduced, the Arrow card was a haste buff. At the time, BLM AoE was something like
Fire3 Fire2 Flare [(EtherHQ/Manafont) Flare] Transpose
and MP would refill enough during the next Fire3 slow cast to continue. However, if you gave the BLM an Arrow, they would almost always finish casting Fire3 before the server tick, ending up in AF3 with not enough MP to cast any fire spells at all, ruining their flow and requiring them to waste time switching back to ice again. A BLM who was paying really close attention might have realized they were hasted and deliberately paused for just a second so that they could get enough MP, or maybe even status-offed the Arrow, but jobs shouldn't have to think that hard about making changes to their basic rotation just because there's sometimes a party member who can haste and throw off that rotation.
Anyway, this specific example is no longer relevant because of the way Astral Fire and Umbral Ice were reworked in DT and because BLM AoE changes since HW; but as I said in an earlier post haste will reduce the mobility of casters that get a fixed number of instant casts per minute, by making their GCD timers shorter, which could affect them negatively during a savage run where they expect to know exactly how much time they have to dodge each thing in the fight script and they expect those timings to be consistent between attempts.