If I were to guess, I would guess that the large number of initially uninhabitable planets that require tech research to be able to colonize them later in the game is a response to the many criticisms the devs have gotten about the initial planet rush typical during the early game period. They certainly took a pounding about it, and I think they came up with a neat solution, although the "realism" of the solution could certainly be questioned.

Personally I think the far end extreme High quality toxic/radioactive planets should be rare or slightly below in appearance of the settings of what you set 'habitable' ones. I prefer settings where Habitable are 'uncommon' . I would like to see the extreme actually slightly more abundant thus giving us as players a reason to go down those lines. Lets face it, any rocky planet outside the Goldilocks area of its star will be either frozen or barren and unlikely to be a water/aquatic one. Mars is not toxic and is really just barren. 

All this boils down to player preference (as it should). I like large voids between clusters of stars. Some players like a smooth distribution of stars and planets in their galaxy. I would like a good start to get empire going then start looking for the 'extreme' ones to extend my credits or research base. 

Oh and the Extreme focus techs need more clear info regarding what I get for those techs. I want to see that Frozen Planets gives me a 15% bonus to research and full production. Also I want to unlock all of the extreme planets as a full production. I do not want a research penalty as described by Gilmoy for doing this one tech line. Others I can get but this seems to really force my hand on what planet I want to build.