Each member of our family has 1 bike. Me and the boys pretty much ride to school./work everyday, regardless of the weather on the same bike, all fixies.

A friends moved abroad recently and left us his old road bike.... I have used it twice once just to see what it was like and once because my kids forgot to replace the patches in the patch kit and had used my spare tube......

I see no reason for a second bike.. sure it would be a nice toy/tool to have if you can afford it. but if I can get by for 15 years using one bike, then I suppose anybody can....