Well...working from my local codebase (Which was updated within the last month), I see:
if ( safeloading_arg.found() ) {
Cmdline_safeloading = 1;
}
if (!Cmdline_safeloading) {
findhandle = _findfirst(longname, &findstruct);
//...
} else
#endif
{
FILE *fp = fopen(longname, "rb" );
Cmdline_safeloading is initialized to 0. And your point that _findfirst causing bugs is exactly why the flag is there in the first place, because it doesn't take into account access issues, etc that fopen would, and it's not definite proof that Freespace 2 can open the file like the code assumes it can. So if somebody has reversed what the flag does within the last month, then they've reversed the point of having it and it ought to be -unsafeloading. But as you pointed out, it's been like this for ~5 years, so I would assume that there hasn't been any serious issues with its use.
It's my code - I haven't checked on it on a long time, hence why I didn't know how it interacted with the 'new' caching stuff, but I do remember that I put the flag in in the first place to answer some of this concern, although I don't remember who raised it.