First off let me say that I buy the majority of my games used, and I subscribe to gamefly so Microsoft blocking used games would be bad for me personaly.
But if Microsoft does ban used games it could mean the end for Sony if they don't.
First off people such as myself who only buy used games are horible customers as far as revenue is concerned. We purchase the system which the manufacturers take a loss on. Then we never purchase any games which is where the revenue comes in for the manufactures/publishers. If predominantly used gamers flock to playstaion, they would be actually taking money from Sony, who is in dire need of some cash. If enough people buy a ps4 and don't buy any new games Sony could loose the profits it stands to make on the ps4 game sales.
Second. The idea to block used games assuredly comes from game publishers, and Microsoft wouldn't do it unless they knew the game publishers would support their decision by giving exclusive games/ features/DLC. As a publisher if you knew you'd make more money off games sold on Xbox then you'd make sure the ps4 version came out a couple weeks later or was less appealing than the Xbox version, in order to encourage more people to buy it on Xbox.
Over the course of the console cycle the ps4 would have to foot the entire bill for non revenue generating used gamers. Sony needs to make a lot of money with ps4 so that could cause huge problems for them.
With all of that said I honestly don't think Microsoft will ban used games. (Maybe something like online passes, but an all out ban seems out of the question) it's a very risky move that I'm not sure they are willing to take. But if they do decide to go through with it, I'm sure it's due to the reasons stated above.