Why is it that MP3Tag defines several genres as different TCON tags?
ID3v2.3 states clearly that several genres should be defined into a unique TCON tag, starting the field definition with a '(', and separating each genre with parenthesis, such as '(Pop)(Rock)'...
TFYH.
It's one of several areas where Mp3tag does not follow strictly the specification by ID3.org.
I doubt anybody follows that particular one you mentioned.
So? Should we say MP3Tag is ID3v2.3 compliant? 
What i mentioned is what ID3 states in rev. 2.3. And, "from time to time", foobar2000 does keep it (it depends on how "non-conformantly" the ID3 tags were written).
Making MP3Tag an strict ID3-compliant tagger would be a big step for human race. Perhaps others would try to follow 
Yes, I know what you're referring.
I'm not quite sure what you're saying. You mean that when foobar is set to "ID3v2 writer compatibilty mode" (i.e. writing ID3v2.3 tags instead of -2.4) it writes TCON frame in a way specified in official guideline - ID3v2.3.0? I doubt it.
Yep, I can just see the social benefits flowing from that single change. Humanity will be freed of all its ills...