Andy Neil wrote:
It has advantages for source code control - ensuring that everything you need is present & visible in one place.

I have never ever some necessity to make source code control for files which are part of standard compiler delivery. There are also libraries as part of installation. Should also libraries be presented & visible in one place?
Keil is not so huge compiler but I cannot imagine to do such source code control for C++ Builder headers for example.

Andy Neil wrote:
It could also protect you from nasty surprises if something gets changed in some mysterious folder deep in the depths of your Keil installation...

Hmm... Keil is not such horrible as you write :-). Really I also had some problems with headers when we switched from V5.xx to V7.xx. There really was small changes in their headers. So I made small patch in my own header. This patch header is part of my project.

Every new SW version can brings some problems.
It is rule which is known to every developer. :-)