On the surface, I like the idea, but I think it would be difficult to do it right. As JT pointed out, managers have to adapt their approach to the players they have on the team. I don't think it would work to define manager ratings for things such as preference to bunt, steal, hit and run, slow hook, fast hook, etc. Same thing for the GM. I don't think a GM should prefer speed, power, pitching, defense, etc. The GM should be drafting and trading to fill positions where there are weaknesses.
For a fictional sim, maybe the way to do this would be to have a GM skill to accurately evaluate all the players in the organization and define a draft/trade requirement. Then you have a scout who's skill is to, well, scout talent. The GM then uses the scouting report to conduct the draft or make a trade. At the manager level, I think like the OP said, use more intangible skills such as player development, game knowledge, etc. And I believe all of these ratings should be hidden, or have the option to be hidden. Bill Bavasi, Billy Beane, Joe Torre, and Bruce Bochy don't come with ratings, they come with a track record.
For a real player sim, don't we already get the real GM's philosophy? If I start an association in 2006 and take over the Oakland A's, I already have a team filled with players drafted and signed by the real A's GM, Billy Beane. I think with player affinity turned on, in subsequent drafts the players will be drafted onto their original teams, right? That is assuming the human controlled team doesn't draft a player that was drafted that year by another team. If I am wrong, let me know. Then I could see a way to add real managers that fit the team at that time. But, even that could be generally defined as a team philosophy for that particular season. The only gap then is trades, and for this, I would have the same thoughts as I do for fictional sim GM's and scouts.
Anyway, this post is getting long. I could go on for days about the pros and cons of a feature like this. I just really don't want this game to turn into something like OOTP. I abandoned OOTP for the simplicity of PureSim. With PureSim, I can have a real player or fictional player association up and running in short order. I like it that way.