COM really isn't that hard to learn. Especially if you just take a COM snippet and adjust it to use $1/$2 as mentioned and then supply the COM object name and parameter. The MSDN site has a list of all the COM objects and parameters and what they do and what kind of output to expect.
Also, stating that scripters don't know how to use COM isn't really a good excuse. If a scripter could learn how to script in mIRC, they can learn how to use COM. In fact, it would take considerably less time (you can learn enough to use it in a day without much problem if you make use of a COM snippet that is already written). In fact, I wouldn't be surprised if you can find a snippet already set up with $1 and $2 and all you have to do is use it and provide the information (again, easily available from MSDN). Then you don't need to know anything about COM other than looking up the object and parameter names.
As far as the problems with OS versions, if it's scripted, you can easily adjust it to handle any differences with OS versions. If it's a built-in identifier, it can only do what it was set up to do. You can't just adjust it when something changes from OS to OS. For example, let's say that $os stops working in Windows 8. Until mIRC provides a fix, your script(s) that use it will not function and unless you know how to use COM or another method of getting the OS, you are going to be stuck. If it's scripted instead, then you can make a few changes and have it up and working again very quickly.
Regarding the "how many" question... Think about all of the different system stats someone might want to display. Let's list a few. Graphics card, graphics memory, graphics cpu speed, monitor resolution, monitor frequency. That's just for graphics. Then you have total RAM, RAM in use, CPU, CPU speed, CPU in use, Motherboard, etc. These are just a small number of the things you might want to display (or others will want to display). Should mIRC create identifiers for every single system info item people will want to use? As you can see, it will quickly become a very large number of new identifiers.
I'm not saying some new ones shouldn't be added. But I certainly don't think we need identifiers for everything. And then it becomes a question of where to stop. I can see graphics information or RAM or CPU being much more widely used that OS build, for example.
