Using the classic BTN PFR vs SB and BB, Minimum Defense Frequency (MDF) is used to prevent Btn from auto-profiting by opening PF. We calculate the bluff odds he is getting; SB and BB cumulatively need to defend enough to prevent auto profit. However, I am wondering if MDF is really an appropriate application for forming our defending ranges. 

What is so bad about letting Btn auto profit? Or better yet, why are we using this criteria as a basis for the floor of how much we should defend? It seems to me that the sole question we should ask is what range of hands can we have better ev than -1bb or -0.5bb for BB, and SB respectively.  If that range is above the threshold for him having an auto-profit open, that just means he shouldn't open 100%. If our defense freq isn't above MDF thats fine as well, it just means he can open 100%. 

Btns potential ability to profitably open 100% doesn't mean we are being exploited or playing sub-optimally. It would be an indication that the positional advantage he has is strong enough such that he can open 100%. We lose nothing overall because in theory of reciprocity, we will all be Btn and blinds equal shares each orbit. 

This is something that just popped into my head the other day. It may be flawed thought process or it may have been covered already. Please share your thoughts and/or enlighten me.

edit: If this forum is the wrong spot feel free to move.