As I say to customers I like my fixed gear but there is no way I would give up my geared bikes. Each one serves it's purpose to the 3rd cousinland (all the other lands have been taken by stupid fascists). I do like the simplicity of a fixed gear (or even a single speed) and there are a lot of cool little innovations making it easier or giving you wider ranges (Dingle cogs, Dos Eno freewheel, disc brake cogs...) but let's say doing a fully loaded tour fixed just doesn't quite have the appeal.

That all being said my main dream bike is a SS/FG cross bike so I guess it says something about me. However I still would have geared bikes and I think having a few different bikes in your quiver is always a good thing. Have a SS/FG, maybe a geared mountain bike or road bike or a bad weather commuter or what have you. One should never limit themselves to one bike or one style of bike.

As far as knees and stuff as long as you find a good gear ratio and don't try and skid stop brakeless fixie cool points, you should be relatively fine. Eat kale/dark leafy greens, almonds, white beans and other non-dairy foods with tons of calcium and you will be even better off. Dairy is known to leach calcium from your bones which is no bueño.