I have very good, even excellent lenses I do not often use. Some are full-frame, retained mainly for 35mm film, which I rarely shoot. Their FL is not what I look for when shooting APS-C. After years of this situation, now Pentax has entered the FF market with a winner, so I am contemplating regaining the proper perspective of these fine lenses. So now I am glad I didn't get rid of them.
I do have some minor but decent lenses I should get rid of, and will.
The excellent lenses I don't often use, some only good for APS-C, include expensive ones, even Limited primes. However, my attitude regarding my equipment is similar to my attitude towards a tool chest, which harbors fine, expensive tools not often used. When the need does arise, the tool there. For instance, I don't shoot macro or near-macro very often. But when I do decide to get into it, or the subject matter at hand draws me into it, I really enjoy what I can get from my DFA 100mm f/2.8 WR lens. The WR allows me to shoot under drippy conditions, which sometimes presents the most charming closeup shots.
Couple of other examples are my top-notch tele primes- DA* 200mm f/2.8 and FA 300mm f/4.5 gems. I got the DA* 200mm back when Pentax was running some special deals, along with the DA* 50-135mm f/2.8 lens. My thinking was I would thus avoid needing a very large, heavy 70-200mm f/2.8. I was correct, since most of my fast tele needs are covered by this zoom lens, and only occasionally do I also need the extra reach. And I only use these when I want extra-high quality and/or its speed for the extra aperture and shutter capability. Otherwise,my very versatile DA 18-135mm does a very good job. And my DA HD 55-300mm f/4-5.8 does a creditable job in good lighting. But when I need the extra-fine tools, I am glad I have them.
Not long ago, I shot friend's college graduation, taking place in a huge pro-sports indoor palace. The family and I were situated in a VIP suite looking down from quite a distance. I hauled out my big guns, taken in their cases and packed into a camera bag. I wound up using all three as the ceremony entailed the need. Afterwards, for family group shots, I had my DA 18-135mm along, which was perfect. The family was amazed at the quality of the shots I gave them. There are other examples as well.
I had a K-r bought new until recently when I gave it to friends, who were celebrating the arrival of their first child. Last year, I had replaced it with the K-S2 at a great price. I have numerous satisfying, and very special images taken with the K-r. But I've found the K-S2 to be a better tool in every way. And, it is still small and lightweight- in some respects smaller even than the K-r, yet with better controls and features, even better IQ, and having WR. So it is now my compact snap-shooter. I got the little kit-lens package for next to nothing extra. I continue to value this little tool because it is so little! Works perfectly with the small camera-so I gave away my old kit lens with the K-r.
My experience says this about your situation. I am rather in parallel with LensBeginner. If you are only interested in snap-shooting friends and family, not needing or ever looking for the extra-high quality the best equipment can deliver, I can see your point. But looking down the road is important. I can tell you, when quality is an issue, I found out for myself that though I was getting quality with my K-5, knowing that being better than what I got with my nice little K-r (which you may have also noticed with your K-30), that when I decided to also get the K-5 IIs (both got at about 1/2 price as outgoing models), I did notice an additional refinement of detail from not having the AA filter! AF was also better. This improvement in fine detail was more noticeable when using very high-quality lenses!
Now I also have the KP. No AA filter (switchable), very high resolution,very good color balance, very good dynamic range, etc. Very good handling for a relatively compact body, and having essentially pro-oriented controls and metal-body. If you had a KP and were shooting with your DA 20-40mm Limited, and looking for quality from scenics, etc. this experience would be very convincing for you, I believe. The more on-body, fully pro-oriented controls of my K-5 IIs still rule, but this is the tradeoff of the KP for being more compact.
I could see selling your 21mm, and 40mm Limiteds. Since you have the 20-40mm Limited, they would mainly be for times when small-as-possible is an issue. And you have the fast 50mm f/1.4 when you need more aperture and added reach. The DA 10-17mm is a fisheye fun lens, a specialty item, but not a linear true WA lens. It is very special as such. So for linear use, the DA 15mm f/4 Limited is very appealing, very tiny, with very good IQ and has low distortion for such a FL- the opposite of the fisheye zoom. Both lenses are very potentially useful for different purposes.
The 18-55mm kit lens is dispensable, as is the 70-300mm tele zoom. I only hang on to mine for film and perhaps being useful on an upcoming FF DSLR. I don't know why you wish to keep the FA 100mm f/3.5 instead of the Tamron 90mm f/2.8 true macro lens, which has more capability. I would sell them both to get my DFA 100mm f/2.8 WR macro!
The price of the KP has been drifting down. You might consider getting one if you are interested in quality, and in taking full advantage of the quality lenses you have! Since getting it, I find myself using my DA 20-40mm LTD, 15mm LTD, and DA* 50-135mm more often!
