i've beaten ifrit many many times as a mage. i can tell you that you are 100% incorrect.
if you are having to cancel a spell during ifrit then you are horrible at timing the spells. i have well over 250 kills on him at this point and you are way off base.
1. vulcan burst: once it blows you back you have to start casting stoneskin while sliding backwards or you do not get the spell completed before his next move in stage 4 or 5. he has 5 secs between abilities and it's a 6 second cast. you have to cast and be running back to the spot while casting or you do not get it off.
2. running from eruptions: there are times that you will dodge all of the eruptions fine, but by the time you get back to the group you will only have a few seconds left to cast cure. you will start casting the cure while you are running back to the group so it finishes going off before the next ability.
3. hellfire: the entire group is moving when you hit your macro to curaga the group while getting back to your location you should be in before the next ability spam.
4: mp regen: you obviously have no clue what things such as siphon mp nor stygian spikes was used for. i move quite frequently in battle and very rarely do i ever need to use the mp regen ability besides those.
5: positioning mob: there's times in which the tank will be forced to move the mob in which it will put him out of range of the healer. when the tank moves the healer must move with him to keep him within range to be able to cast a heal.
there is such things in a battle called situational things in which you will be REQUIRED to either move or die. those things and being able to cast while doing so is the only reason groups stay alive. as a dd i understand you don't know what you are talking about, but there's the truth.
it's like telling a dd he cannot use a taunt or use a tp move unless he is sitting still or that if he moves all of his buffs go away and he can't use a tp move..