I always has this in my mind using only OCS as the call manager (without using any PBX system). This question has makes me to setup a lab with OCS 2007 Front-end server and Mediation server and a simple media gateway. In the lab I've tested the with 2 types of incoming lines (PRI and normal PSTN lines). I would said anyone can try out this setup via normal PSTN line.
In the lab I can see the difference of using the 2 types of media gateway. Using PRI would be more direct and simple configuration on the media gateway (as media gateway guide has it all). But when using PSTN line media gateway (i guest is lower end), there are more configure step required on the media gateway.
Using the lower end media gateway there are some featues likes call ID (caller phone no.) was not able to pass over to OCS. I've checked with some media gateway expert, it seem the information (Caller ID information) would not pass over due to the limitation of the devices. I still exploring the possibility of using the lower end unit to have the same features as the PRI line.