1st of all...
There is no free 7zip compression library, yet. Decompression isn't the problem with 7z, rar, etc., compression is.
2nd of all...
with the current prices per gigabyte there is really no need for further compression.
3rd of all...
you may gain some bytes using solid archives...but on the other hand it nearly makes it impossible to remove/replace files...and decompression single files can be very nasty too
and last but not least:
it's not supported by mame ;)

Roman Scherzer
ClrMamePro