There are really two reasons why the ability selector UI works as it does on the consoles, one technical reason and one design reason.
1.) On the consoles, having more than four abilities mapped for instant use at any given time would mean having to have all the memory loaded for every one of those abilities at all times. On the PC that's no big deal, but on the consoles, memory management is tricky to say the least. We could have sacrificed memory somewhere else (by having, say, less enemy variety in each area) to make that possible if need be, but we chose not to due to the other reason, which is more design-oriented: 4 abilities accessed instantly via the trigger menu simply worked better than the umpteen other approaches we tried over the course of development.
2.) We did a TON of user testing with all manner of different UIs and control schemes for accessing abilities during Reckoning's development, from lists to radials to multi-tiered menus (at one point we even had abilities arranged in a dpad menu much like the weapon menu for Half-Life 2 on console). We tried having UIs that pause the game and UIs that didn't, etc. D-pad, triggers, bumpers, you name it. In the end of the day, this approach proved to be the most effective at being A.) Easy to learn, B.) Easy to operate, C.) Not slowing down the pace of combat, and D.) Granting the player access to enough abilities at a moment's notice so that you can employ a nice mix of magic attacks in a single fight without having to pause the game and select a new spell to make active. In short, we went with this approach because we found in practice that it works extremely well.
That said, if you find you want access to more than 4 active abilities (and you don't want to play on PC), there are three factors that mitigate the issue:
1.) As I noted in an earlier post, our spells and weapon attacks include quite a bit of depth simply by virtue of how we handle contextual combos and player input in general. Storm Bolt and Tempest, for instance, collectively require only a single active ability to be slotted rather than being independent abilities that have to be slotted on their own. In short, two weapons and four abilities doesn't mean you've only got 6 total actions you can do in combat--you could easily have 20 things you could do with that toolkit if you've unlocked some special attacks and ability upgrades.
2.) Sustained abilities can be mapped, turned on, and then unmapped to free up a slot. It'll still use up its mana cost as long as it's on, but it won't take up one of your four mappable slots.
3.) Remapping abilities is pretty speedy. Press Start, select Abilities, select the ability you want and hit X (or square on the PS3). It's not something I ever find myself doing in active combat (I never need to), but if you wanted to for whatever reason, it's not a particularly time-intensive process.
