According to *ist D specifications (
http://www.pentaxusa.com/images/products/istD_Specs.pdf), it says "K mount lenses usable with restrictions". It's commonly believed to be in the same league as the other 4 film bodies (MZ-50, MZ-30, MZ-60 and *ist). As such, there's a trick around the "metering and shooting only at wide aperture" problem. Don't mount the lens completely, and it effectively becomes a manual diagphragm lens.
You might be interested to know that if you had bought into the Canon FD mount 16 years ago, it would be obsoleted that very year in 1987. The first EOS camera was launched then, probably at a price tag few students can afford.
Nikon's physical mount has not changed, try mounting an old series F-mount lens on a some new Nikon bodies now and see if it works. Olympus OM mount has also been recently been declared obsolete by the company, without a replacement SLR mount other than the digital-only four-thirds system.
Pentax has faithfully been serving its customers with unparalleled mount compatibility till today. With the exception of 4 bodies (or 5, if the *ist D proved to be unable to meter with K/M lenses), all other bodies work 100% with all K-mount lenses made from 1975 (that's almost 30 years ago). Which other brands gives you that? The closest would be Nikon and its vast lens system (both AI and pre-AI series), but you have to be fortunately enough to own a good, old mechanical Nikon body.
For a company to survive, it's understandable that they will move on and introduce more gimmicks to earn more money out of consumers. You have to decide if you will want to be sucked into the scheme of things.