The Yamaha XS650 has no water pump. The Yamaha XS650 has no fuel pump. The Yamaha XS650 has no sophisticated computer or electronics. The Yamaha XS650 can be repaired without sophisticated tools and test equipment.
The only negative I see is that you have to be willing and able to keep it repaired by yourself. We have XS650.com to help with that.
That's why the correct number of bikes to have is n+1, n being the number of bikes you currently own.