Originally Posted by
StrawberryZebra
In theory, you can fit exactly 8 GCD's into a Bahamut summon, one per 2.5 seconds at base. In practice, your GCD is going to be shorter than 2.5 sec because you've got some Spell Speed so you're going to have a small gap at the end where you're not currently casting a spell, yet could squeeze in another instant before Bahamut ends. So an extra spell rather than just the 8. It doesn't specifically have to be Ruin 4, but you're able to make sure you have one available when you need it rather than relying on a proc.
It gives you a lot more potential options in combat. They might not all be good options, but they're there at least. No longer being tied to a proc means you can gaurentee it's availability when you want to use it, and that's a good change that rewards careful planning and forward thinking in my book.