As I said it is where avast scans files, should you not exclude it at worst it will just cause duplication of scanning, at worst MBAM locks the file whilst it scans it.
If it is a file that is infected and mbam alerts on it, then it will try to deal with what is in essence a temporary file (not the real one). All of this stops avast from scanning the file and won't deal with the real file as it won't have scanned what it was trying to scan.
This could let malware into your system rather than prevent it as it could be something that is coming down from the internet (via the web shield), not good.