Why the defaults for tunable kernel parameters matter
Yesterday I ranted in passing, as I have before, about Unix vendors that have never adjusted the defaults for any tunable kernel parameters and thus leave parameters set at values that were perhaps correct fifteen years ago on much different machines, but which certainly aren't any more. Given that tunable kernel parameters can after all be changed to better values, it may not be obvious why this is a problem.
There's two reasons why you should get tunable parameters correct from the start. The first is that people will get artificially and unnecessarily bad performance until they learn enough to change the parameters (if they ever do). I suspect that many people never change the defaults and instead go along either vaguely wondering why their system performs badly or simply thinking that this is the intrinsic speed of your system. If this seems irrational, well, note that there is generally a large collection of tunable parameters, many of them are badly documented, and then it's far from obvious which parameters (if any) are applicable to your problem. Especially if you don't have detailed knowledge of the kernel.
This leads to the second problem, which is that the OS vendor's kernel people will normally know a lot more about the meaning, effects, and sensible values of kernel parameters than some random sysadmin will. Having the system tuned by the latter instead of the former is not something that makes much sense; the former is going to do a better job (and sometimes a significantly better one). This is not a new or novel problem; instead it's the same old problem you have any time the experts punt hard issues to non-experts instead of dealing with them.
(In fact many sysadmins are going to use whatever settings a quick web search leads them to. This is sort of okay if the top hits of such searches have sensible advice, but you lose badly if the top hits lead people to bad advice.)