What is the compelling reason to thin the herd? If your reasons for riding are narrowed down, the question of the kind of bike you need answers itself. For many years I had "only" 2 bikes, a racing bike and a touring bike. I got away from riding much for a while, and I was down to just 1 mountain bike. Getting back into it as a hobby and riding for fun/fitness is when the sickness started for me.

But a friend of mine, who doesn't drive, gets by just fine with a hybrid. All the immigrant workers I see in my area ride around on dept. store mountain bikes, and I doubt they get up in the morning and wonder which bike they should ride to work.

If you have the space and the money, though, keep everything and buy more! There are common themes around here that selling leads to regret, and everything is getting more expensive and harder to find.