I actually thought that I WAS afflicted with multiple bike syndrome, just a different strain than the one you're referring to. My garage is packed to capacity with various 2-wheel contraptions. It's just that for me to fall in love with a bike, it requires owning it for a while, getting to know it and having it take me through many memorable experiences, not taking a brand new one out for a test ride.
Loyalty? I don't think it's loyalty. It's just that with time I grow comfortable with things that surround me, and then have a difficult time letting go. It's same with my old friends, with housewares, tools. Just a trait of character, I guess.