Two reasons. First, because abilities in 4e are more varied than 3.5, and the tactical situations which call for them are more likely to come up. For example, I'm playing a Warmage with a bonus to fire spells in a campaign right now. My third level spell list, for all intents and purposes, might as well just be Fireball. My second level spell list, likewise, could be Scorching Ray. 90% of the time, that's all I'm using--and 90% of the time all I'm interested in is doing the maximum damage as quickly as possible. One of the design goals of 4e was to balance out the spells so they can achieve tactical objectives, and there's going to be more choosing between Fireball and Blink, and less choosing between Fireball and Flame Arrow.Lanefan said:Why won't people use their best ability - whatever it is - as often as they can?
Second, because your best ability is only usable once a day. Your second best ability is only usable once an encounter. Sure, you might open the first battle with your best attack, but then you've got to hope the thing you killed didn't have a mother waiting right around the corner. The per day and per encounter abilities impose an opportunity cost--per encounter a small one, per day a large one--and that's going to force players to make a decision about to use them.