Originally posted by monochrome Sigma lenses are clearly not a beneficial part of the K-mount ecosystem. Tamron lenses, for instance, have a symbiotic relationship with the Pentax system.
Sigma lenses have a parasitic relationship with the K-mount system, as a leech,
This is your opinion, not a fact. And you are certainly entitled to it, but now that I know your mindset, I understand where your comments are coming from.
I have the opposite opinion. I see Sigma as a valuable part of the K-mount ecosystem, and I feel that without the additional lens choices that Sigma brings, a Pentax DSLR would not be nearly as compelling. While there is some overlap between Pentax and Sigma lens choices, both companies also offer some very unique lenses.
For example, where is the Pentax equivalent of the Sigma 8-16mm? And where is Pentax's silent-focusing 85mm/1.4 portrait lens? And where are the 30/35mm f/1.4 lenses from Pentax? And of course there's the big 150-500mm and 50-500mm Sigma twins. Where is Pentax's affordable 500mm zoom lens? It's only very recently that they came out with the 150-450mm. And while that's a very good lens, it has a more narrow focal range and is priced far outside the budget of most hobbyists, which is where the bulk of Pentax customers come from. And for those who do have a big budget, Sigma offered the excellent 500mm/4.5 long before the Pentax 560mm/5.6 was released, and the Sigma may still be the better choice.
And even when there is direct overlap, the Sigma may still be a better choice. For example, you have the Pentax 50-135mm f/2.8 vs the Sigma 50-150mm f/2.8. Both lenses are fantastic optically, but the Sigma has more reach, focuses quicker, and is not plagued with the problematic SDM focus motor.
And to give Pentax their due, they also offer some unique and wonderful lenses, such as the Limited line. And the sweet 10-17mm fisheye is another unique offering. The way I see it, the Pentax lenses appeal to a different user than many of the Sigma lenses, so together there is something to suit almost every taste. So that's why I find the hatred of Sigma hard to understand.