I'd buy it to mess around with, but the actual use cases of them are real marginal. There's a number of systems that you'd think would be a use case, to do an end run around their broken design, but it often turns out that the design is multiply broken and there is no improving them. I am pointing a particular finger here at Dell/HP/Compaq 775 systems from about 2004-2008, not all of them, mostly the low end compact ones. Some of those have a nasty quirk where you can't use PCIe graphics and more than 2GB of RAM... they may have onboard slug mode graphics... So you'd think that PCI graphics would help you out, but nope, not as primary graphics adapter, or may not be recognized at all. I have a 9400GT PCI that I have tried in several systems like this and they hate it. Meanwhile other PCIe/PCI motherboards it works fine on. I think some of this is that BIOSes will look for graphics in 2 places only, and onboard always takes one. It can happen with socket 7 or slot 1 with onboard, PCI works for graphics but it won't initialize an ISA graphics card. Whereas same chipset, no onboard, it does.
What makes those Dell etc frustrating, is that CPU wise, you can often get fast Core2 duo in there... backed up only by extra lame onboard with 4GB, or crippled to 2GB of RAM and some later PCIe cards won't even work, so you think "yay, PCI will save the day" but nope. So a number of systems that might appear to be a use case for these, aren't. That's why I think that late PCI cards often turn up "barely used in box" because they got bought and tried and were unsuitable.
Prior to those, you're often looking at them for Socket T, socket A, socket 478, socket 754 boards that you can cram a lot of CPU performance onto, but have only onboard graphics. With these you're thinking, "skip a few gens and the low end card is equivalent to a higher end card" which can be true and satisfactory. However these types of boards tend to be less satisfactory in other areas, using slower RAM, and only 2 slots to fill, and other performance un-hancing problems that make them less than useful for "lower end of XP" gaming, where these late PCI cards are useful.... and start you thinking more of "top end of Win98" where these late PCI cards run into compatibility problems. At that point the FX5x00 and ATI 9xx0 PCI cards start looking a lot better... and the DX9 benefit isn't all that great for late 98 either, such that settling for 8500 or 440MX PCI or so isn't crippling them much (And most of them can be played on dx7 dx8 cards with lower eye candy, but FAR more acceptable speeds than slow dx9 HW support)
So anyway, all I'm saying is, buy it to screw around with fine, but buy it to solve a particular problem, and unless you've borrowed a buddy's card to see if it does solve that problem, then you've got a low chance of being lucky. Of course there are also those of us with a few "problem" boards/systems where you can tailor the problem to suit the solution.... but I put that in the "buying it to screw around with" category.
edit: there's probably an actual actual use case in using it for some 2000-2010 simulation, flight, space or racing, when you've run out of PCIe slots to get more than 4 displays and you still want another one for another view or control panel or something.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.
