Mark CMG
Creative Mountain Games
[MENTION=5](. . .) over the course of his career (. . .)
I love your underlying idea for many reasons but this one quoted phrase causes me to dispute tying passive learning to a system that increases the bonus level-upon-level. For my own part, I have worked with a system that allows players to justify increasing their bonuses by substantiating increases (even mid-level) based on actual situational factors (and there are limits to just how much any person can learn in any given period, passively or actively).
For instance, if much of what gets accomplished between level four and five takes place in or on the ocean, a player could certainly justify that they gained some skill in swimming, even if the actual time at the table doesn't specifically involve the character actively trying to learn to swim better.
To use an example similar to your own, if the characters are in the city much of the time between levels two and four, and for expeiancy's sake we choose to allow equipment purchased from the list between sessions, certainly a player would have cause to say their character has become more streetwise and familiar with the ins and outs of purchasing from merchants, perhaps even extending that reach toward other social skills.
In this manner, passive learning in-game becomes actively implemented, meta-game-wise, for specifc circumstances that are clearly justifiable, IMO.