This is not an option, I do have 3x 951`s, 2011 and recently bought ac^2. Ac^2 have only 5 ports, all of them are occupied, and I occasionally need to switch one more device (100Mbit), which is now done by switching my old Linksys with DDWRT, however, it's not an option since it's an only 100Mbit device, so I am betraying 1Gbps link to switch that external device. Buying another mikrotik router for this purpose is nonsense. Btw, DDWRT has great support for both those USB-ETH adapters.MikroTik have enough small and cheap APs that you can use without such an external USB ethernet adapter.
E.g. when you want a mAP 2n with one more ethernet port, take a look at the hAP lite.
It would be great, but really, I highly doubt that this will ever happen...Mikrotik will hopefully add support for plethora of peripherials in ROS v7 ... and I'm genuinely looking forward to see it in not-so-distant future.