I currently have 4 bikes, each of different ilks, and the next was a progression of the bike before it.
I'll mention just the bikes I have now. The ones before were all sportbikes.
The TLR is a great around town bike. I took it on a 3500 mile week long trip to Florida. That gave me the sport touring bug, but I realized that I needed something a little more comfortable.
Enter the Busa. Picked this up 3 months after the TLR Florida trip. GPS, satellite radio, hard bags, and 70,000 miles later, it's the perfect sport tourer for me. Over the course of 70,000 miles and 49 states on the Busa, I got the itch to ride to Prudhoe Bay, Alaska. Can't do over 800 miles of gravel on the Busa easily, so it was time to go shopping.
Enter the KLR. It will suck to ride 10,000 miles in one trip on, but 800 miles of the trip this year, the KLR will be the perfect tool for the job. It's actually fun as hell to ride, so I'm pretty sure I'll keep it and explore some more off-road around here.
While doing 49 states on the Busa, I had time to think about what my next touring type bike would be. I specifically decided I wouldn't get another touring bike until after I did all 49 states on the Busa. Just a little thing I told myself. I polished off the full 49 states last May. So, time to think seriously about the next bike. The BMW came along at the right place and the right time, so I snagged it up from Troy. It is an amazing bike, and I fully expect to continue my touring on that bike and rack up a ton of miles on it too.
Each bike has a defined role and purpose. At least, that's what I tell myself to justify owning 4 motorcycles.