As far as I can remember, Chikyuu has always used both Necro AND 12. Even in that old B5 exhibition video you can already see him get streaks with 12. All the other vids I’ve seen show him using both characters alternately. It seems he’s been using 12 more in SBO2 qualifiers recently though, but I don’t know if that really means anything.
I didn’t personally ask him about his choice (although if you’re really interested I can try next month at Absolution), but given how good he is with both characters, it doesn’t really matter which one he choses to stick with to be honest… He looks like he can beat anyone with both, and Necro really isn’t that much better than 12. Whoever considers 12 crap doesn’t know 3s very well IMO, he’s just harder to learn than most characters but definitely not crap.
As usual, the whole Necro fad probably has more to do with the current top Necro Japanese players being freakishly good more than anything else. Chikyuu’s probably using whichever one he finds most fun at the time. As for using low tier characters, it’s not like he’s the only one doing that. You’d be surprised at how many (good) 12 players there are in Japan, like Moto or Pistachio. People don’t know about them because they don’t travel outside of Japan. Personally, I don’t think Chikyuu really cares about how “good” his characters are considered, didn’t he use Raiden in CvS too?