User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170420000500/@comment-23853100-20170421104101

"‘Extra Demons’ are famous devils who are not from the 72 Pillars. Among them there is Mephistopheles and other ridiculously strong devils, and they hid themselves in the depths of the Underworld, making a certain distance with the current devil’s government. Only a countable number of their clans have survived. Even the status of those from Grayfia’s house is unknown apart from her. The ‘Extra Demons’ are just only for settings currently."

It says current not former Devil's government. It means they participated in the society when the Original Maous are still alive.

Purity by devils will became more less important when they got mixed with reincarned devils.

What are you talking about? Why would it get less important because they're getting less. It's the opposite. Pandas are being protected because they're few in numbers. People want to make sure Panda continue to exist so that they can be shown to the future. Same mentality here, the Devils will want to make sure they don't run out of Pure-blooded Devils to pass on to the future.

So tell me how can it be Devils won't care about purity anymore? In fact, try giving me a single statement where Rias, Sona or any Pure Devil who outright claim that Pure Devils isn't important.