It's more of a what was decent instead of what was cheap.
About 70% of the Maat fights revolved around 2hr abilities. About 50% of the CoP relied on 2hr one point or another. Most Zigart fights were also first relied on 2hrs. Most BC's were walked in expecting to have a 2hr in your sleeves.
It's the opposite of what wasn't cheap.
BRD- considered cheap
PLD -always an ace in the hole
RDM - always an ace in a hole
COR - next to godly if used correctly
SMN- almost a key tactic in soloing summons or generally BCs definitely cheap
SAM - The most youtubed
BLM - Come on it was good before now now they have mana wall...
SCH - manafront and chainspelling...certainly cheap
DNC - Basically like SAM. massive damage
WHM - After they fixed the enmity full HP for entire party all around decent and something you want in a tough fight.
BLU - A little like ES a little like Mightly strike. It wasn't useless but then BLUs were already powerful. Useful for low acc skillls like 1000needle and useful for unlocking limits like drain kiss. Still cheap.
The odd man outs at cheap but not so much BC:
Ninja : death warping, but also good for baiting, or saving PTs
Thf : To Run away is so like them
DRG : It's life as a drg, the 2h timer needs to be there.
RNG : decent at lower level, though it's ben tweaked a few times.
Warrior : yea the lower end of 2hrs
2hr is a good timer for them. It ensures no one ever attempts to rely on it, only make a risky bet on it. That's the way cheat abilities should be.
10minutes: Coffee break
1/2hr means a round
1hr : Possible, but still kinda short
2hr : Most people consider it a session. So it's the right limit instead of 24hrs.
2hr was very strategic. They wanted to make it a "one day" or some super long time like FF14's levequests but then ran into a problem of casual vs hardcore. No one expects to wait 2hrs, but they also have it their when they go "adventuring".
They probably considered a session of "adventuring" about 2-3hrs on average. Which still holds relatively true even today.