There are a lot of people saying "If all powers of level X are balanced, there should be no cost (or less cost) for swapping them out!"
But this completely ignores the fact that powers are balanced
in context. I tried to get this across earlier, with my "fighter with an area attack" example. The fact that wizard power X is balanced with fighter power X assumes that wizard power X is taken
by a wizard, and fighter power X is taken
by a fighter. They may remain equal out of that assumed context.
For example--and I'm making this up, not offering a hint of what's in the book

--an encounter power that says you can ignore the effects of a successful melee attack made against you is more valuable to a fighter than to a wizard, because the fighter's in melee a
lot more often than the wizard is. (As an actually in-the-book example, consider the possibility of a fighter acquiring
mirror image.) Similarly, a utility power that says you can throw a ranged or area power without drawing an opportunity attack is more valuable to a spellcaster or an archer (obviously) than to a primarily melee-based fighter.
My point? Powers are balanced
in their assumed context. That doesn't mean they're balanced in a vacuum, and that there should be no cost for swapping them out.