Certainly fast cast works with potency gear for spells that have a long casting time, but the criticality of timing depends on the situation. When things heat up, everything adds up. And perhaps I'm biased because I play support more often than other roles, but I think this is particularly true for support.

If I'm main healing on mobs like caturae I need to shave every second off my casts that I possibly can. They throw out nasties rapidly, and I need to be able to respond quickly. Half-seconds are significant - they absolutely matter. And that's not just true of Cure - any other spell I'm casting is a cure spell I'm not able to cast in that window, and the longer those other spells take, the longer people have to wait for critical cure spells. Do I cast Haste with some fast cast and land it in maximum duration gear, to reduce the number of casts I have to make? Or do I cast in full fast cast gear, making my next cure available more quickly but requiring more casts over the fight? Cheaters don't have to worry about these decisions.

In another scenario, have you seen those BLM that -ja down the normal Omen mobs on floors 1 and 2? They need their spells to cast as quickly as possible so they can snap back into damage taken down gear almost instantly. Again, small differences in time here make all the difference in how successful that strategy can be.

3+ strong magic bursts in a single window? You need fast cast and potency. And these aren't the only situations where split second timing matters.

The argument that the game isn't really designed for these scenarios is reasonable, and on some levels I agree. But whether the game is designed for it or not, people who cheat can do it and people who don't cheat can't do it. With my Cure example, obviously the best solution to the problem is to bring more healing power. But why do that when your cheating healer can cope, and maximise on the timing gains with other cheating tools? Why bring two BLMs to magic burst when you can bring one to triple burst and avoid an Escha monster's HP increase?

And when everyone in the party is just that little bit faster at landing their full potency spells (or whatever), of course it's going to make a difference to overall performance.

For me this issue is not about the necessity of maximising fast cast and potency simultaneously. If we're honest, it's not necessary - you can bring more people or change the strategy. The issue is that some people can do it, with all the side benefits that brings, and some people can't. The solution is to either stop people cheating, or enable everyone to play at the same level.