I already had several non-Ortlieb top tube bags, and I wanted the convenience and added security of a quick-release system that would allow me to bring my valuables into a market with me and didn't require me to fiddle with rethreading and adjusting straps each time I got back on.
The Ortlieb Fuel bag and Top tube bag use the same mounting system. Ortlieb offers the Ortlieb Mounting-Set to support the use of the same bag across multiple bikes. But if you are handy with an awl or hot nail, you can modify the bags you already own.
This system works very well in retrofitting other top tube bags. All the hardware you need to attach the quick release to your bike is included. I converted a Timbuk2 bag to take advantage of the M5 frame bosses. The included internal bag fasteners provide enough rigidity to remove the four frame and stem straps from the bag and rely solely on the bosses.
I no longer have to worry about:
1. straps wearing down the bike frame and stem finishes
2. a top tube bag flopping from side to side
3. leaving valuables or an expensive frame bag on the bike when I head into a store or overnight accommodations
Finally, if you pick up a couple of these, you can quickly move one bag between bikes and swap bags on your bikes depending on the ride's needs.
