Also, do that many people really use THAT many abilities EVERY encounter. Even on the harder difficulties I never found it necessary to use more than 5 or 6 for the most part.
I don't always use 8 abilities in one fight*, but that's because I don't know in advance what opposition I'm going to face. If I'm fighting many small enemies, I use my area of effect spells. If I face one strong enemy, I use my debuffs and spells that target a single enemy.
Mike Laidlaw said the new system encourages planning in advance, but as I said in another thread, I don't think we should be able to plan fights in advance. Dragon Age: Origins had encounters with opposition you didn't expect, and it required taking your time to look at your abilities and making smart choices. For instance, that one optional fight in the mountain cave where you're fighting a bunch of cultists, two drakes, and a mage who deflects all ranged attacks who you can barely reach due to a large number of traps.
If a situation like that were in DA:I, then it may turn out you brought the wrong abilities for the job. You die, have to reload, and remap your abilities. This is tedious and not at all fun. The only way BioWare can counter this particular problem, is by not surprising the player with unexpected opposition, which would take away an element of challenge.
Besides, In DA:O and DA2, I used 6 slots for potions, grenades and poisons alone.
*Edit: I just looked and at the end of Awakening, I had 18 abilities (not counting sustains and consumables), of which I used 11 in almost every fight, with a twelfth that I used in more difficult fights. This is as a warrior.