I'll put in a word on the pro-fast-cheap-prime side. Models like the K-x aim at drawing new people into the system (a LOT... I never heard students wanting a camera mention Pentax before, but now the K-x pops up in every discussion), and the many photography-enthusiastic students I know go out and get a cheap fast prime (usually a 50mm) first thing to put on their new cameras. Some of them who are bargain shoppers even skip the kit lens and just go with a prime.
If it's got decent optics (again, how hard could it be to just copy the optics from the M 50mm F1.7 and just update the coatings?) at a good price (in line with what others are charging for their entry-level fast 50s), a lot of people, including that newer market that the K-x drew in, as well as experienced Pentax shooters like myself, will buy it regardless of how cheaply it's built (can't be any worse than Canon's F1.8
). They could probably even weatherseal it cheaply given the lack of moving parts and have an extra selling point.
R+D would be minimal, it would be cheap to produce, and I can almost garauntee it would be one of Pentax's top sellers within a year or so. It would also help hold on to users who eventually leave the brand due to a lack of cheap, fast primes (which has been happening since the price bump). It would certainly ship far more volume than a lens like, say, the DA* 200mm, not to mention Pentax's other higher-up lenses, and it would keep people in the system so that they could aspire to the higher-end lenses over time.
I don't see what's not to like. Offering an industry-standard product like a cheap fast 50 does not lower a brand's prestige, it just boosts their image by making their lineup more complete.