Shang!
I totally understand your feelings. I only tried the Tamron 28-75 among the lenses you mentioned, which is a superb lens, but at the end I decided to keep the FA* 28-70 costing almost 4 times more and sold the Tamron. I just love shooting with Pentax glass and not entirely just for the IQ (which also is sliglty better in this situation).
I also agree that one of the major perks of Pentax ownsership is their glass. It's not only their peerless limited series (both DA and FA), which gives unparalleled size, speed, IQ, price advantgae that no other system can privide. But, I used the first EVER SLR lens released by a Japanes SLR company (Asahi) on the most modern Pentax DSLR - The Asahi-Koagku 50/3.5 the lens that came with the first Japanese SLR to be ever made can meter, have SR and be used very well on the K-7, HOW COOL IS THAT...now, every lens in between can be used as well - what other system can match this!
Now, assume none of what I mentioned in the previous paragraph is true, and all Pentax has in it's lens collection is just ONE of the FA limiteds - then that is reason enough for me to be a Pentaxian - Yeah! they are that GOOD
But, I would not go far enough to say that all you should have are Pentax lenses -Zeiss, Voigtlander, Leica and some others have strong offerings that are in some cases even better than any Pentax lens. But, there are some consumer third party lenses that are worth owning as well. I own the Sigma 100-300 F4 and the Tamron 90/2.8 macro. I like them both, but I am severly tempted to replace them with Pentax equivalents
...there are some others as well (Tamron 28-75 is one of them for sure). But, I would never call any of them better than Pentax counterparts (if there is one).
P.S: Can you tell I am a die hard pentax fanboy