I don't necessarily agree with this, I understand the principle but if you have 25 instances with fixed eCPM floors with 6 ad networks , it's all about the availability rate of each instance.
Then it's normal that some instances will provide very low fill at certain eCPM, in that case you can either reduce the floor or deactivate that instance.
The bottom line here, in my opinion, is that more instances are set, more chances there are to get a higher eCPM.