The benefit is that it can run in dual-socket motherboards. The first QPI path is for connecting to a chipset, and the second one is for connecting to another (identical) processor installed in the same motherboard so that they could work together.
Here's the Intel's documentation about your question (pages 25-26). A brief rundown:
1) Both CPUs must have similar TDP;
2) Both CPUs must have similar QPI and RAM speeds;
3) Both CPUs must have similar number of cores and threads;
4) Both CPUs have to be based on a similar micro-architecture;
5) Stepping does not matter;
6) Clock speed does not matter, either.
It turns out you won't be able to mix your X5680 with any other lower-end Xeon because the closest CPU to yours in the Xeon tier list is the X5675 which comes with 95W TDP.
Probably I wasn't clear on it, but I think the X5675 wouldn't work together with your X5680 just because the X5675 is a 95W TDP part and your X5680 is a 130W TDP part and that contradicts the first statement I have posted above. Anyway, if was going for a dual-socket setup myself, I would be getting identical CPUs anyway and I suggest you do too.
I ran a x5660 and a x5650 in an Asus dual socket.
I found when all cores were working on 1 work unit from folding at home they worked at the slower CPU speed.
When running 12 work units, 1 WU per core, they worked at rated speed Running.BOINC
If I remember right in an EVGA SR2 a team member was running a hex core and a quad core for 10/20 cores/threads.