I have a variety of motivations for what's in my fleet, but all considerations start with proper fit and ride quality. The latter usually means Reynolds 531 DB steel, clearance for 30mm or larger tires, relatively steep geometry--at least for the go-faster bikes. But I think my motivations are best described by the bikes that haven't made the cut:

Peugeot PX-10: I've had and refurbished at least 3, but never felt the ride was anything that screamed out keeper.

Raleigh Professional: I do have one in my fleet (a 1971), but it's likely the bike to get moved on this year. I've had at least 3 others, but like the PX-10s, the ride quality wasn't anything special.

Raleigh International: I also have one of these in my fleet, a 1971 that's been fully gugie-fied and is a 650B conversion; I've had 3 or 4 others from 1970-74, and they all rode fine, but didn't ride better than what I already had (particularly my Rivendell Romulus, which is essentially based on the design of the Int'l, I understand).

Peter Weigle: I bought a frameset on eBay, an early 90s one before PJW got into rando builds; this was more of a race-y bike. I liked it fine, but for what it would fetch in re-sale, I liked just as well two or three others in the fleet that cost a fraction of the Weigle.

'73 Raleigh RRA: This is a rare bike, built only one or two years and featuring an all-French drivetrain. I didn't like the way it rode at all.

'50-'51 Raleigh Clubman: At one point I had a 1949, a 1950, and a 1951 Raleigh Clubman. I've kept the '49 and moved on the '50 and '51 (which are quite different designs than the '49). Really, it's crazy to hoard.

a couple of mid-80s Mercians: Nice bikes, but superfluous at the time and easily found again in the resale market.

Many Raleigh Super Courses: I have a strong affection for this model as it was my first real vintage road bike when I purchased a used one in 1983. Since then, I have bought (usually as frame sets), fixed up and sold many, but usually have one in the fleet. That present one is a '73 with Capella lugs set up as a 5-speed IGH with drop bars.