Well I am a "IFR" pilot and I prefer it high in my turbo w/ o2. Having said that you throw up a random number 15k which is beyond the limits of some planes. It is different for diff. planes and situations of course. I guess I should have explained if there is a 100kt headwind at 10k ft and you have a cub it would probably be better a little lower.
That's my point. But it's more than just headwinds and flying at the highest altitude that the plane is capable of (as you're aware, but now I'm going to elaborate).
For example, the Aztec has horrendous cooling over the engines. In the summer at above 7,000 ft, it's difficult to keep the cylinder head temps reasonable without dumping ludicrous amounts of fuel in, even with the cowl flaps. So my options are fly higher (which the plane will do) and risk shorter engine life (not to mention the fact that the plane just won't fly happily up there) or be much more comfortable at a lower altitude (we'll ignore the option of dumping ludicrous amounts of fuel in, which is not ideal for a number of reasons). I'll take the lower altitude. The plane and I will both be happier. Then there's icing, for which there are lots of reasons to fly at various altitudes, high or low depending.
Similarly, I can take a 35K departure from PHL on a busy Friday afternoon in the summer or say I need full length. Then they'll probably taxi me to 27L, which means crossing 35 (probably at K), then crossing 27R, then getting to 27L, and waiting my turn. Meanwhile, my oil and cylinders hitting ludicrous temperatures, and my passengers and I are baking in the greenhouse of a cabin. Let's say I'm doing an Angel Flight, and one of those passengers therefore may have certain health implications due to sitting in a greenhouse of a cabin that I might be able to safely tolerate. Nevermind the fact that, as my cylinders are baking and my oil is boiling, the engine may idle just fine, but who knows what will happen when I push the power forward? Now I run a higher risk of a fowled plug and other potential pitfalls. When faced with that option, I chose 35K, just like the DHC-8s ahead of me filled with passengers.
Let's say I'm flying a Cheyenne, and now I have air conditioning and pressurization (not to mention turbines). Ok, I'm less worried about my temps now, and my passengers and I are comfortable (well, depending on just how hot it is...). However I'm burning 200 lbs/hr combined idling on the ground. For reference, my fuel burn at my cruising altitude of FL200 will be 400 lbs/hr, with roughly 2800 lbs of Jet A total at my disposal. That extra 30 minutes on the ground equates to losing 15 minutes of fuel at cruising altitude. So even if my passenger and I are comfortable, that still doesn't necessarily mean that it's a good idea. I may want that 15 minutes of fuel, even though I don't push my margins close. Turbine fuel burn characteristics can make planning interesting, especially when you get unexpected curveballs.
While I don't think anyone doesn't prefer long runways, flying high, and with full fuel, there are times when the runway behind you, altitude above you, and fuel in the truck make sense to be in those places, and can contribute to a safer outcome of a flight.