I can't argue with Marco, Harvey, or Ed; their logic is spot on.
With that said, I've been running a Weltmeister short-shift kit that I bought used from EASY in ~1991 and installed during a lunch break at Costco. The car lost a transmission in ~1992 when a driveshaft knocked the starter clean off the bell housing. That hurt, especially when making $8.27 an hour! The short-shift kit stayed and a used 901 (unknown miles) out of a nice 914 went in. 28 years later, that 901 has only been apart to fix leaks, maybe 5-10 years ago, and John Holleran commented it didn't need anything other than seals. I've used the car as intended over 75,000-100,000 miles, and it's still one of the best-shifting 901s I've tried. Some of that is "every 901 is a little different and you know yours," but it's more than that. Others have also commented on how nicely it shifts, too, and I credit that to a healthy 901, fresh bushings, proper adjustment, and, yes, the Weltmeister kit.
For me, the stock shifter's throws were too long and prosaic. The stock shifter is easier, but I found its action to be too light and a bit vague. The stock shifter was and still is in really nice shape, so maybe I'll put it back in just to try it—as my impressions were probably formed with worn bushings. Or not, as I have no complaints with the way my 901 shifts and have seen no detriment to longevity. The mechanical reasons against the Weltmeister kit are obvious and hard to argue against, but I have to wonder if use doesn't play a role? I can see where shifting a stock 901 linkage too quickly or not heel-toeing through the gears might exacerbate wear and tear with the Weltmeister, but I've always been (relatively) slow, gentle (two-finger pressure), and deliberate and have no grinding, no tough-to-get-into gears, and a nice enough action that I have repeatedly dismissed the idea of "upgrading" to one of the fancier setups. Also like that the Weltmeister setup remains hidden under the carpeting—something the Tuthill setup does too.
My guess is: A lot of Porsche drivers are just hard on the machinery. I see it all the time in my industry, and sometimes cringe. Redlining an engine with cold oil, slam shifts, etc. Learning in a 1964 bus that was my dad's daily driver for 30+ years was an exercise in preservation…he likened driving it to flying an Eastern Block airliner back in the day: You want to grease every landing. It was good advice, and I don't think you end up much (if any) slower over road or track. But YMMV!
What I've always wanted to ask mechanics: How often do you go for a ride with customers to observe how they drive their cars? I suspect the takeaways would be pretty interesting...