Sun Workstations, sure, I can see a collecting niche for those; a workstation is a "tactile" thing meant to be seen and used directly by humans and there's a lot of nostalgia value to be had in immersing yourself in their unique UI quirks. The problem with SPARC servers is, well, over a console connection by and large old Solaris is an awful lot like new Solaris, be it on a Sparc or x86 box. (And to a lesser degree, UNIX is UNIX; unless you were running some specific custom/vertical market application that you can't recompile you were probably using it to do something that's easy enough move to something newer with minimal disruption to your workflow. This is of course why Linux on commodity hardware has essentially destroyed the commercial UNIX market outside of some very specific niches.) It's sort of like the difference between collecting classic cars vs. old mining equipment or dump trucks; sure, the latter might be "interesting" but you need a huge garage and it's far less likely you'll impress a casual observer with your collection without having to do a lot of explaining about *what exactly* makes each item special.
Of course, I say this despite totally thinking it'd be cool, if I owned the Biggest Mansion in MansionLand, to have a big PDP-11 or early VAX in a room in the basement running some archaic version of BSD 2.x. I dunno, maybe give those SPARC bricks another 20 years and I'll reconsider.