I've wondered about it too - but not to the point of giving up the ol' 550F.
I've only been back into bikes since April of '09 - after quiting in 1960. Favorite nephew got me back into it [he rides a very nice '04 T-bird and admits the '77 550 handles better than his T-bird].
We went to look at a nice CB450 and there sat a good, all orig CB550-Four, faded light met green that the guy said could be bought cheaply. We got very interested! However, instead of selling it the owner had it shipped to him in Calif - from Chattanooga! We felt like it was "the one that got away".
Now somehow "hooked" on the 550-Four's, I began searching for another one - mostly on Craigs. Found 4 "rebuilders" and finally found this one in Knoxville -up and going but with "needs". In our ignorance, we likely paid $450 too much. Then, tires, a Carb job, etc.
I've got more in the ol' bike than it will bring. I haven't and wont add it all up. However, it now runs great [thanks to the great guys on this Forum], it is now trimmed out to suit my addmittedly redneck taste [small Shoei's, crash bar, windshield]. So, the cost doesn't matter since it ain't for sale.
There are so many points to really like and enjoy about these ol' 4-bangers!
It must be we like well-built vintage hondas, excellent 4-cyl engines, good handling, etc.
Did I say "vintage"? We obviously enjoy the challenges they present [at times, all too frequently]. The nostalgia, the pleasure of having something different - not run-of-the-mill.
I think it is a "feeling" we share for these bikes that can't quite be put into words. It is likely at least a slightly different motivation for each individual. If it could be easlly described, we might not llike 'em as well. But we DO like 'em!!!