Is there a reason you can't record such ideas in the "comments" section?
As I said in the OP...
"I realize there is a freeform comments box and I currently attempt to use this (sometimes) to track this sort of information."
However, I list reasons why having actual fields would be a significant improvement over each user developing their own personal system using the freeform comments box. Really, it's the same reason why we have any of the other fields. Technically, game length, number of players, who played, etc...could all be tracked in the comments box. The data would be significantly less useful and significantly less convenient to record. For these same reasons, custom fields would be a very nice feature.