Let's look at Support DPS...

[Rasikko]

Does every buff have to add damage? Maybe give BRD something unique and new. How about haste? Obviously good enough where it doesn't screw with GCDs.

[Duskane]

Does every buff have to add damage? Maybe give BRD something unique and new. How about haste? Obviously good enough where it doesn't screw with GCDs.

adding haste would still be adding damage in the long run
and no it doesnt have to be about damage but people generally tend to ignore none damage boosting utility unless its PLD

[HyoMinPark]

Does every buff have to add damage? Maybe give BRD something unique and new. How about haste? Obviously good enough where it doesn't screw with GCDs.

The only issue with haste is that some jobs will not like it—I believe this would currently apply to NIN, MCH, and DNC. Not sure how SMN and RDM feel about Haste, but I’ve never heard them liking spell speed, so.

It would depend on what haste applies to. Auto-attack would likely be fine. But if it directly affected the GCD like AST’s old Arrow did, it likely wouldn’t be that popular save for with certain jobs.

[CKNovel]

The only issue with haste is that some jobs will not like it—I believe this would currently apply to NIN, MCH, and DNC. Not sure how SMN and RDM feel about Haste, but I’ve never heard them liking spell speed, so.

It would depend on what haste applies to. Auto-attack would likely be fine. But if it directly affected the GCD like AST’s old Arrow did, it likely wouldn’t be that popular save for with certain jobs.

Haste could indeed desynch some rotation, MCH tends to go with a very fixed sks for the best Heat generation and Drill/Air Anchor cooldowns. One good example is that 2.48s GCD is the "norm" but 2.44s GCD is best for Levi to squeeze an extra Air Anchor before the first Maelstrom.
NINs and SAMs have their fixed cooldown at 60s, extra sks would desynch their cooldowns, especially TA.

It would lead to more complex optimization, "What is my best GCD in this fight with this support".

The final goal is to deal damage, so damage is the best kind of support to go without annoying any other jobs.