What works for me is that each one of my bikes has its own personality. They (finally) all fit the same and I'm positioned the same on all four. Although they each have a primary role to fulfill, there's a good bit of overlap between them. I'll ride any of them to work (and I do), or on a club ride (and I have). While any of them would work on a century, two of them would work better than the others. Each brings a little something different to the table.
The Portland just oozes confidence and competence. Right from the first mile of the test ride, I knew that the bike would go wherever I pointed it, without complaint or fanfare. In the ride and handling department, it's the bike equivalent to an S-class Mercedes. Handles like a sports car, rides like a sedan, feels solid as a bank vault.
YellowBike is frisky. It reminds me of a puppy. Open the door and it shoots out. All it wants to do is run and play. That bike wears me out.
Blue Steel is a bit of a dichotomy. It's a crit bike--handles like one, and performs like one. It's a real hoot to ride, particularly in the cut-and-thrust of city traffic. But the bike's personality is all happy and sunny. It's just as content to poke along in the single digits. And it climbs like the dickens.
I named the Litespeed "Jeeves" because it completely disappears, then when it's needed, it's right there, having anticipated my needs. It has a reserved, understated way of communicating, much like the stereotypical English butler. It's quiet in the main, but when the road goes "BUMP!", Jeeves reports, "Pardon me sir, but there's a bump. All over now. Sorry to have bothered you." Although it has yet to bring me tea.
So for me, it's not what components they wear (there's everything from Sora to Dura-Ace in the mix). It's not that they're all fast, or all handle well, or all ride nice. It's not their sizes (56, 57 and 58 are represented) or their geometry (that's all over the map) or their material (two are aluminum, one steel, one titanium). It's their certain intangible "personalities" that makes me like them.