In my very own opinion, things like preventing movement while casting is more of a balancing mechanic than something to cancel an action (and moving in XI wasn't even the best way to cancel longer cast time spells, healing and re-standing was).

It's a small thing but something that I believe (depending on how the battle mechanics will be set up) will be something that is important to the game balance. Since everyone is still always so keen to use XI as an example, I'll do the same.

in XI, if BLM didn't have to stop to cast their Damage spells, what would be the point of Strategic magic like Bind and stun? BLM for those that used it correctly was a solo Powerhouse, some of the things they could do (or more importantly a group of them could do) in terms of End game content was madness(kaeko anyone?). Not having to stop to cast would have made this far more of an issue.

Taking this into XIV, in which we're in a transitional phase from 1.0 to 2.0 - As I see it once you removed the animation lock that these servers have once we implement 2.0 would basically be the above scenario. Instead, they are taking away our ability to cast while moving now (even though I'd say being able to cast while moving now because of server lag is still a joke if you think you can) so that when everything rolls around for 2.0 we're not bitching and complaining then.

The way I see it:
Moving while casting = less potent spells to compensate for this huge advantage it gives you.
Not moving while casting = Strategic placement of your body to optimize the situation without compromising your abilities.

In Rift, the most important spells that a caster could use all required you to stay stationary. And that's another one of those PvP heavy games.

It's all a balancing act even in the most minute and overlooked ways.