You see, you have that backwards. Yes, the current Parallels version runs on older OS versions. But the older versions of Parallels do not run on more recent OS X.
Which is what has turned this into essentially an annual subscription model, for anybody who adopts Apples next release.
Okay, as numerous others have pointed out, that's not quite true, and I retract it in part. But it's purely a timing game - it's entirely possible to have to enter at a point that requires paying twice within the first year, even within just a few months. And maybe again in a year, depending on where in the more-or-less 2-year cycle you entered. Happened to me at one point, has happened to many.
Two years ago I calculated that, since autumn of 2008, it had cost me a grand total of $152.25 to keep two Macs (one is running OS X Server) updated. And $499.16 in Parallels + Quicken + Windows updates, just to be able to continue to launch a functional version of Quicken on one of them. (And since then, $19.99 more to Apple, an additional $94.57 to run Quicken... which actually omits the $49.99 I "saved" by skipping a cycle and running Boot Camp instead for a year.)
Current status: I'm still on Windows 7. Quicken 2014 will need an upgrade in a few months to be able to project taxes. Parallels under Yosemite, still a question, though likely also about to need upgrading.
All I've ever needed was the ability to continue running Quicken. I run no other Windows program and haven't needed a single new feature from Intuit or Parallels (or Microsoft) over the entire period. For nothing more than basic continue-to-run-just-like-you-did-yesterday functionality, even updating only when needed rather than every release, comes to about $100/year.
The funny thing is that I'd happily shell out every few years or so just to keep modern, or for performance improvements. But instead of EVER doing a .1 compatibility release - a fine customer relations move - these folks' business plans center on milking their customers, year (or two) after year after year. By forcing me rather than giving me a choice, they earn my disdain rather than my loyalty, and it certainly looks like about 9 of 10 forum posters feel similarly... a simply stunning business model.