It's unusual. Typically MAME collections are distributed with deterministic compression and using automatic managing tools, so people can stay up to date without re-downloading large amounts of good data. 40GB torrents tend to hash 4MB chunks at a time, while a ROM might be only 8KB-- there's a high probability of a chunk becoming invalid.
When someone updates to the next MAME version, they can download an "update" pack with only the ROMs that changed, and a tool will recompress them together with the old files so a torrent client can start seeding the new version immediately.
When someone updates to the next MAME version, they can download an "update" pack with only the ROMs that changed, and a tool will recompress them together with the old files so a torrent client can start seeding the new version immediately.