I think I just want to be able to move any work (mostly text-based stuff) that I compose on my vintage machine to other places, so email would do the trick. (Also buying a vintage machine makes a little more practical sense when you can connect to it virtually. Although a private network is another possibility?)
For slinging text around you don't even need a network card. A null modem cable to another box works just as good. Kermit is a standard protocol/terminal emulator package that supports file transfer over the serial ports. Versions are available for pretty much every OS and computer combination.
Also, if you have an SE FDHD or an SE/30 (or anything else with a HD floppy drive, LC series, etc.) you can read and write DOS formatted floppy disks with no problems on the Mac. (Early macs use 400k or 800k floppy formats, which are compatible with nothing else). All Macs from the Plus on up have SCSI, so a SCSI Zip drive makes file transfer easy too.
If you're a UNIX geek, old compact macs make for great little terminals - you can cable the Mac to a PC (running Linux or BSD) through the serial port and have the Mac act as the console terminal. Therefore you can do anything text-mode UNIX can do through the old Mac. You can then easily compose things on the Mac (Say, in Microsoft Word for Macintosh or ClarisWorks), save the file locally, then Kermit it to the Linux box and use the Linux box to email it - all while sitting at the Mac's keyboard.
Lots of options. If, for example, you are happy using text mode *NIX, you could run a Linux box headless (say, a small form factor PC), and have it cabled to the Mac using the Mac as the serial console. You could have a perfectly workable computer, capable of doing anything Linux can do from the console (Pine for email, BitchX for IRC, Lynx for web browser, tin or mutt for news, emacs, vi or nano for word processing, easy access to FTP, etc), and the only interface would be, say, a Mac Plus. All text mode, of course.
Macs are nice because they're common, and there's a lot of software for them. And, the classic little all in one models are so cute :D
-Ian