I think a lot of why this stuff is not touched upon in the forums much is because it's hard to tell weather it's really vintage or not. Vintage PC and Vintage Macintosh are a lot different from Vintage Computers in general, it's sort of a sub-genre.
The older stuff is more collectible because it's harder to find and harder to do something with with their strange old 6502/8080/Z-80 processors, hand-built memory cards, lower resolution video, lower Kilobytes (4K-64K) memory, and weird media formats (like 720K single sided 8" floppies in a box as big as a washing machine). Most of that software cannot be run on a modern PC without some sort of Emulator, and most of the hardware is not capable of too many of the modern tasks without someone to make a new application for it.
However, old PC's are possibly the easiest thing to collect. As soon as IBM's first one hit the shelves, it's set the standards for everything today. At one time just about every big corporation had a plethora of IBM XT, AT, and even some original PC doing all the number crunching for them in some way, during the 80's, often interfaced to an AS/400 or more often a 360 Mainframe for file storage and data interchange. However, some of those machines are special for this reason such as....
The IBM PC 5150 - The First PC
The Columbia Data Products MPC 1600 - The first decent IBM PC Clone
The Compaq Portable - The first Luggable PC, as well as the first Legal (okay, gray area) clone
The IBM PC XT 5160 - THE standard that was copied by just about everyone at some point
The IBM PC AT - The first 80286
The Tandy 1000 Series - One of the most popular computers of the 1980's
The Compaq Deskpro 386 - The first commercially available/successful 386 based PC
The IBM PC 5140 - A very interesting portable bridge between the Personal Computer and PS/2 series
The IBM PS/2 Model 25 - The first of the IBM PS/2 series machines, and one of the few 8086 machines
The IBM PS/2 Model 80 - IBM's most well known and hugest computer sizewise
On top of it, there are scads of historical pieces of hardware and software that make these machines one of the newer collectible deals around, not to mention that they are cheap as heck to get your hands on if you know where to look. Also, they are all a lot like snowflakes, you can score 4 IBM XT's and not a single one of them will have the same configuration. Each machine seems to have a story to tell, weather it was a file server, a BBS surfer, a family machine, primarily used for games by someone's kid, or was at one time a big business machine interfacing with their Unix based mainframe. And the nice part is due to this nearly anonymous base configuration deal with these older PC's, it makes it a lot more acceptable to put them to frequent use and modify the snot out of them as they are not all that rare except in a few very rare cases.
As for me, part of it is nostalgia, the other part is that I fell into vintage computers by my want to have a computer in the early 2000's, and not being able to afford a new one. So I patched together an 80486 running Windows 3.1 using every resource available to me to find parts, work around minimum system requirements that were untrue, and basically assemble a computer 10 years out of date using cast-off parts and building an experience the whole way. Through all that, I learned a lot, and found it fun to putz around with vintage hardware and wax nostalgic via playing those old DOS games in their natural environment. There's also the challenge of showing what can REALLY be done on something so old, on one hand they are obsolete to the average end user, but not nearly as much as one would think.