Because it's interesting?

I personally would really wish to see a XRL C,bit or JNE C,bit,addr... (at one time I missed it badly when I couldn't really afford all the wasted cycles of JB,JNB,JB,JNB...)
But I've got a table of '51 instructions and there's not too many holes... (note most of Rn and short absolute (ACALL, AJMP) addressing modes take 7 separate instructions)
Right now I see one hole at A5 marked as "reserved" in AT89S252 data sheet - does anyone know what happens if one triggers it?

Now, for undocumented SFRs, there's a whole bunch of them everywhere. Manufacturers put whatever they desire in them, and I wouldn't be surprised to find some easter eggs, unfinished buggy implementations of ideas erased from data sheets but not from actual hardware, enabling/disabling "hidden features" available only in more expensive chip versions etc...