Ultimately... Movement interrupting spellcasting is a matter of preference.

Being able to move while casting allows for more dynamic fights where the party can be always moving
Imagine a fight where there is a big behemoth moving towards a city to attack it and your party had to kill it before it got there. With movement interrupting spell casting, you'll never see this

When you have to stand still to cast, the fights become more static
This may sound bad (and I would prefer more movement in the combat)... but its technically how classic final fantasy works.... You have generally always posted up in a spot, Hunkered down and fought your heart out.

So let's try not to argue these 2 points. because honestly.. Yoshi said he wants 3 types of spells
1. Spells that you must stand still to cast (Moving will interrupt)
2. Instant cast spells (Moving can't interrupt)
3. Spells that you can cast while moving (Movement wont interrupt)

So this leaves us with 2 issues that will be fixed at 2.0, the only problem being that 2.0 is still a ways off
First issue, some people just wont like movement being made to interrupt spells. But having certain spells that you CAN cast while moving would go a long way to ease their pain... Yoshi is just dead set on adding half finished systems and features into this game. Why would you add spells that are interrupted by moving and not add the ones that can be cast on the go? Maybe he enjoys watching people rage for months only to find that the change fit into the big picture in the end.
Seriously though... think about it...
Yoshi said : "Right now Movement will made made to interrupt spellcasting but in 8 months to a year from now, there will be spells you can indeed cast while moving"
Its kind of goofy if you ask me

The Second issue is the current server structure is admittedly GARBAGE. Adding something that hinges so importantly on latency was probably a bad decision until the server latency is in check. We all have to agree on this.
Also, Yoshi... I know you're new to the Public relations game... but you can't say "Gamepad users will be fine if they aren't worried about fast spell casting"
I mean, I guess you CAN say that... but expect a shit-storm as a result.

tl;dr So ultimately... come 2.0, we'll have a lot less latency and spells that can indeed be cast while moving, and I think that is an incredible compromise between traditional static casting and newer more dynamic fights. There is just so much time between now and 2.0