Much of which I think captures how most fans might have imagined it working...and then thow in a rule that says I can walk and then talk, but not talk and then walk (just as an analogy).
As I've said before, I think this interpretation is backwards. Changing the game to allow having not just move+move and move+QA, but also QA+move
(and also QA+QA, there's as little thematic justification for not allowing this as there is for not allowing QA+move) would break so many things that this can't have possibly been something that was added late in the game development. It
must have been added early. It's part of the very foundation of the game that everything is built upon, just like physics are built around arbitrary numbers such as pi, e and g.
So asking "why can't I QA+move" is, to me, akin to asking "why didn't you make a completely different game instead?"
As for what the reasons were for this early decision, only the designers can possibly answer that. This sounds plausible:
A: "Hey, let's make a game, where two mages battle each other"
B: "That sounds neat! They can move, and summon, and cast spells"
A: "Yeah, but summoning should take longer than casting, right?"
B: "Descent has these neat half-actions, where you can move+attack, or attack+attack, or attack+move, we can take that and have complex spells take both half-actions"
A: "Hm, that sounds cool. Let's see how this goes, I've made this prototype"
...some playtesting...
A: "Yeah, that looks good, but we should make it impossible to cast two spells for non-mages, and movement didn't feel like it was actually important because you could just attack and then move away"
B: "I have this neat idea: We allow just one cast, but mages get this QuickCast marker thingy, and everybody has to move before casting to make movement actually have an impact on the game"
A: "That sounds neat, the game will actually
work if we do that, movement will be important, quite unlike in our playtests! Let's start building this thing and make thousands of people happy!"
