Why the "if any", when you report that PMMDiags "actually uses that stuff"!? I don't know why PMMail on its own would ever "actually use those settings for anything" anyway, unless such an app would be some extension of PMMail.
PMMDiags uses it because it was the easiest way to find where the user installed PMMail. It was also a way to get people to actually set the defaults, but very few people use PMMDiags, so it probably doesn't matter anyway. PMMail could use it to find out which browser to use, but it hasn't been programmed to do that, it is still a setting in the PMMail program (mostly because there is no way to ensure that the default browser, or the user, would do the proper browser setting).
The basic flaw with using the WPURL settings, with PMMail, is that it can only point to one account, when Mailto is clicked in the browser. PMMail could override that, and offer the user a selection from the configured accounts, but somebody would need to do the programming. I expect that other e-mail programs would have the same restriction, unless they are programmed to handle it. There has been no demand to do it, and programmers are hard to find.
There are other WPURL settings, for other programs too (FTP, News, IRC). I don't know of any program that actually uses them, and since it is unique to OS/2, programs from other operating systems don't know about it. In most cases, there is really no reason to even consider using those settings, so they are rarely actually used.
unfortunately the install procedure of an OS sets a different default e-mail client.
In fact, the OS installer does not, and should not, set any of those things. The program that the setting points to should advertise itself by setting them. PMMail will do that, when it is installed, but the user has the choice, so it doesn't always happen. ArcaOS has included a tool so that a user can do the settings, without having to directly edit an INI file. There was an older program (REXX), that did the same thing.
Note that Firefox, Thunderbird, and Seamonkey do set them (apparently), when they are installed (perhaps only when they are set to be the default program). PMMail gives the user the option, but it IS a user option, it is not required, or done by default. If people think it would be a good idea to just do it, I can arrange that (in future versions of PMMail). Just let me know, but that would be a problem, if the user wants to use more than one e-mail program.
it's not as easy as setting a default browser, and e.g. MAILTO.EXE and YARN.EXE are e-mail solutions which may be hard to integrate.
If they work, in a way that can be integrated, you can change the settings to use them. The main "problem" is that the implementation of the WPURL stuff is totally inadequate for the way that most users would want to use it. This stuff is NOT officially a part of OS/2. AFAIR, it was added by a user (Chuck McInnes, I believe) many years ago. It was mostly ignored, although the idea does have merit.