If I'm moving around using WASD, I can't be using any spell with a cast-time, anyway. If I'm casting a spell that is cancelled by movement, I can't be moving anyway while I press the button to trigger it. So, cast-time spells can be bound to movement keys using modifiers to toggle them (ctrl, alt, shift).
I don't think I'll try it, but he does have a good point. I just don't think I need so many keys that I need to overload the movement keys. Here's why.
I use Bartender to get four rows of action bars. My top row maps 1-5, q, e, and r, giving me eight keybinds I can use without moving my hand off of WASD. My second row has shift-1, shift-2, etc., giving me eight more keybinds. My third row is for control-1, etc., and my fourth row is for control-shift-1, etc. This gives me 32 keybinds that are conveniently reachable from without moving from WASD. It's plenty.
By doing it this way, I hardly ever need to define macros. A lot of people write macros to make shift-3 do something different from control-3. This is more flexible, but it's more of a nuisance. If you use multiple action bars for the different modifier combinations, you can set them up using drag and drop.
Note that I left alt out of my list of modifiers. Adding alt to the mix would mean there are 64 keybinds available. However, I find the default meaning of alt to be convenient. It cuts down on the keybinding space significantly, but 32 is really already enough.
Stepping back from all this, I'm left with a big question: why doesn't Blizzard make the defaults better? They could give us four action bars by default, giving you a new bar every 20 levels or so. When we get a new spell, they could drop them into a reasonably place by default. Some people say that Blizzard can't do this because every user is different, but I call baloney. Blizzard knows very well what buttons people mash, and they know it better than most players. They've proven this by having the buttons light up when they are a good option to cast; why can't they do the much easier task of giving us good defaults?