Your eyes already create a natural motion blur so i can´t see the point of having it turned on.
especially because the MB of AC is avery very good one.. not that overdone thing of for example pcars or crysis lol... where it is like beeing drunk or so..
the AC one is very subtl or well when screenshooting not very subtl but it just behave realistically like a cam... so it's a nice thing to have it for replays or even much more for screenshots.. but indeed not needed and also not realistical for driving..
however yes there are problems with it... I think.. when I turn MB on it's not only a big FPS eater.. but also does some strange edgy frame stuck/repeating
also I updated my nvidea driver (which made the complete AC TP feel a lot lot more finished .. also the problem with MB got less but it's still not nice)
with the old driver it didn't matter if I set mb to 1 or to 12.. it always dropped from about 80 fps to 30 with this droping thing to standing picture every second...
now with the new driver.. it drops from about 96fps.. to about 60 or 70 on level 1 (but still with that standing/stucked picture thing) and the higher I turn it on the lower the fps.. down to about 30fps on level 12...
however updating the nvidea driver is a must for all who didn't yet... usually I am not so after driver updates because most of the time all games run good and maybe there is just a slight fps increase or so... but also already had trouble after an update where the driver was bad and I got stutters and lot of fps loss and such thing... so usually I am going with the don't touch a running system behavior on that...
but in this case.. it is an extreme improvement... that I went up from 82fps to 96 fps or so.. is just the tiniest improvement... the whole game feels much more smooth... much more constant fps,,, don't know but it feels like it even look a little bit better... also the whole AA stuff and so have less impact... can run now with AA, antisubtropical filtering and fxaa maxxed at constand and smooth 62fps... and don't feel any lag... before I was getting a slight lag when using fxaa