I'm going to list a game which did a good job pulling it off having different games in one though it's sports, that is, Kinect Sports for Xbox 360. I played it and was pretty addictive as it functioned quite well (give that I had enough space to move around).
I'm not familiar with it. Is it like Wii sports? A selection of games you can choose to play? If so, that doesn't really count - you don't *have* to play the Baseball game, for example, before you can progress to the next round of bowling/tennis/boxing/whatever. Where as if you stick a fighting level after every 3 platform levels which you must complete to progress through the game then it can become quite an annoyance. This is why I don't really like bosses in platform games; they kind of feel out of place.
Special Stages, or optional subgames are a different story - so long as they can be skipped, and they don't lock out anything major from the main game, then they are the perfect place to go off-genre
But overall, I'll agree with you.
Awesome