Which DAO abilities were useless?
Your statement only makes sense if there's a clear dichotomy between having fewer abilities, and having lots of useless abilities.
So, just looking at DAO's mages, did they have "lots" of useless abilities? They certainly had lots of abilities.
I only play mage in Dragon Age (support mage because NPCs don't do support well enough, they can't even tank on their own >.>).
In Origins there was no way of powering your abilities, like I would have loved to power up the buffs you get in the Creation tree, specifically the party member buffs. DA2 was better that Origins because I liked the branches of the abilities, you made the one ability stronger, reduce the MP cost or reduce the cast time.
I always put my most used spells next to my left hand on the WASD keys, or the 1,2,3,4 etc (depending on whether or not I use the WASD to move). Every other spell not next to your WASD keys are a little wasted space. You click on them rarely.
Since you only really use a certain number of abilities continuously, why do you need a bar filled with abilities you rarely use?