There are a number of good reasons for owning more than one bike.
1. If one is broke you still have a bike to ride.
2. If you will be taking one bike on a long trip, you can save the tires on that bike by riding the other.
3. Bikes are different . It's nice to have some variety in the riding experience.
4. One bike just can't be good at everything. Multiple bikes let you choose the best or most fun bike for any given ride.
One problem is that it can sometimes be tough to choose which bike to ride since that are all fun. That's a good "problem" to have
Pretty much everyone I ride with has more than one bike. Some have even more than I do. Once you own more than one bike its hard to imagine only having one.