Delta's wrote a good post here that has a lot of people on the forums and blogs talking.
To me this concept is a no brainer. A well designed mini-game within a RPG greatly enhances the core game. What drove this home to me is Live-Action Roleplaying.
The big problem of a LARP is manpower. You have dozens or even hundreds of players there for 48 hours+ of a weekend event. You never have enough manpower to cover every player for every hour of the event. We learned that if you come up with interesting games within a games you can entertain a lot of players for little need of manpower. One example is the ritual system where players can create magic items and other lasting effects. It require components. The event is setup so that nobody has everything they need. Thus forcing them to interact to trade for what they are missing.
For me the big three for a fantasy RPG are Mass Combat, Estate Managment, and Trading. The first two, Mass Combat and Estate Management, relate to the end game of my campaign. My end game usually involves the playes estabilshing themselves. Currently I use GURPS Mass Combat, and Harn Manor. For D&D variants I will use BattleSystem 1st edition and Expedious Retreat's Magical Medieval Society.
For trade I will use Expeditous Retreat's Spice Road. It is a comphrensive sourcebook on long distance trading for the d20 system.
The major difficulty of adding games within games is their design. If they have any type of detail then you have to essentially design and playtest it along side the core rules. It is very easy to say to heck with it and just focus on the core rules. It is seductive to design a universal system to cover all the mechanics outside of the core rules. However I find universal system to be too generic. When I have to apply to it a specific situations, like running a shop, I having to design it myself.
Mixing and Matching Classes
40 minutes ago