Yes

I want a workstation, not a system that's continuously having things begin/stop working

Fix A, breaks B, etc.

I also want to be able to install something slightly older for not so great hardware and have it be useful.

Yes a lot of people do need it. And by desktop use, although I wasn't clear enough, I mean software development too.

As for the "experimental" and "rapid changes", I can assure you it is far more stable than Ubuntu by miles and miles. Just don't install it at the first day of its release; wait for a month or two.

No change is appreciated by many. People used XP because it was the same experience overall (tweaked by updates). A lot of people (without access to legit installs / copies of recent versions that are even worse copies, or lack of awareness) still use XP. It is the easiest (read: most familiar) version of Windows in existence even today. LTS is the equivalent of that.

Yes. Now I regret I upgraded my Ubuntu to 15.04 Since there is no support for cuda from NVIDIA, so is the Intel graphic driver. If you are using an OS without third party applications, I think it should be fine, but that is not the most case.

Ubuntu provides less than a year of support for non-LTS releases. Anecdotally, this is quite annoying because the overlap between two consecutive non-LTS releases is normally a couple of months and their quality control for new releases is not great.