I see your problem (and proposed solution), but I think that the danger of this is that potentially some evil developer/terrorist/"1337 h4x0r"-wannabe will program the installer to delete the Windows directory, etc. in the instruction set. It would be like a new class of virus or something.
Basically, I think that your proposed patch has a valid purpose, but the potential misuse of the function could have disasterous results. We also wouldn't want NSIS to get the reputation of being a "virus package" or something.
Just my 2 cents.