sunrisekid said:
(surprisingly long debate, skipping most of it)
To respond to the OP, and in support of the early comment regarding calculus retention, I would also add that the body forgets how to perform specialized athletic tasks. Real world "exploits" that, once trained to a high degree proficiency, will attenuate with time and lack of consistent training.
For example, if someone spends a year learning to rock-climb and then stops for a year their body will return to the original gumby state it was in prior to training, regardless of how much time they spent lifting weights.
Anybody who has not taken the time to develop specialized athletic competence is unlikely to understand this.
Another example, related to the early comment on calculus. Anybody who has learned a second language as an adult, then stopped using it, then tried to use it again later in life, certainly understands this phenomenon.
It's kind of pointless to debate the issue of relearning skills in a game when it's precisely the sort of activity that occurs in real life. Of course, disregarding reality is largely what makes D&D fun but...
I would be fine with this rationale if the skills system didn't work in exactly the opposite way, where you can remember and perform every easy DC you have ever learned.
...
The way that I might handle this is that the number of encounter/daily powers at a given level is a hard and fast limit. In addition, you don't unlearn your lower level spells, neither are you limited to any one power per encounter/day, meaning you can use any combination of powers up to your encounter/daily/utility limit. In this way, every class is more like a 3e sorcerer, which is both good and bad, depending on your take.
Good:
>Characters have the option of casting lower level powers at higher levels, but it uses up a "slot" for the encounter/day.
>Higher level characters don't forget powers.
>Characters have more options in combat.
>Characters can use the same ability twice or more an encounter (why exactly can't I use an ability twice?).
Bad:
>Characters may spam the same powerful spell over and over again.
>Characters have more options, meaning they can choose the right ability for the encounter, making the game easier overall.
The good part outweighs the bad for me, but I definitely can see how it could be detrimental to other gaming groups.
...
A less drastic change would be just to allow the apprentice ability and the ability you retrained into "stack", meaning that they share the same slot, and you can choose to cast one or the other but not both. This gives higher level characters the option to cast the previously forgotten lower level spells at the expense of their higher level spells.