I am having Metrix Motorsports rebuild my 3.38 diff since some bearings were making noise. It has been suggested to me that I purchase the Rogue diff cover. Is it worth it since I mainly auto-x and do a few DE's a year?

Always worth it.
Keeping the diff cool is very important.

There are other places that $400 should probably be spent 1st on - engine oil cooler, sump baffles, safety equipment, brake lines, etc. Diffs just don't get that hot in a 20-30 minute DE session or on a 1-2 minute autoX session.

BJ...Matt might have one at his shop...He got an extra one and i'm suppose to get it, but i'm holding off as i might go with a diff cooler.