1. You can ride backwards & other stuff.

2a. A hill climb is a challenge and you'll burn more callories.
2b. A good way to strengthen the legs.

3. Mechanically sound / less wear on components / cheaper in the long run.

4. My bike has become lighter.

5. Most SS are steel and thats a nice material for riding.

6. Like building a kit car....the d-i-y aspect appeals to many a folk.