I guess the thread title says it all... why?
I think that more unique techs would add variability to the game and fun, so I put all the rate techs from all expansions in the bag...
I am concerned though in case I am missing something, how would a 13th rate tech break the balance? I don't think it can be due distorting too much the VPs gained through tech and the discounts, those are minor in the grand total, and having 16 rare techs in a 6p game is still on average less of an impact as having 12 for 2 players... Just wondering here...
You might want to read through this thread:
The topic has been extensively discussed there.