I have not done too much with preferred servers, So take my response in stride.
Preferred servers are basically mirrors of the core. So, they allow for distribution to be parceled on the WAN. Maybe think of a preferred server as being extensions of core.
MDRs can be anything on your network that has an agent installed. We have one or two MDRs on each subnet on a WAN. When you are using MDRs for multicast, you are using their cache to spread out distributions. Once that rep. machine gets the distribution, others get it from that MDR first, and then each other. They don't fetch from the WAN unless MDR is not available. That's why you can set two different bandwidth percentages for Multicast. One for how much of the WAN you want to tie up, other for the LAN.
If you have all your machines in one location, MDRs become less necessary. But our WAN is across the state, so it makes real good sense. WAN Traffic is limited to Core (or all preferred servers) to the MDRs. Ones it gets to the local network, traffic is limited to the LAN.
All this is in ref. to patch deployment. That's all I really do. I have no doubt that if you want to push out computer images and such you would not that happening across the wan if you could help it. Then, Preferred servers make much more sense. You need IIS installed on all preferred servers. All you need on a MDR (Server or workstation) is 1. Agent installed 2. a machine that you know will be on when distribution goes out.
Hope that helps.