I stand corrected.

 >     I'm not sure there are such formats these days. 
 > mex still exists.
 > cs(la)tex still exists.

I don't know about mex (what is that?) but is cs(la)tex doing anything other
than hyphenation support and font stuff?

 > Not saying they are completely independent of latex (they aren't), but
 > they do exist as separate formats.

actually you are right they provided some hacking into input encoding support
in case of cslatex if I remember correctly (but then perhaps I don't) but most
if not all of that should be doable via a package rather than via a
format. Hpyhenation is a different story though.

 >     But even if, the question remains what you gain.
 > Indeed.

:-) but perhaps there are