To my knowledge these packages are not needed at all they should be built into their respective packages for example the libreoffice group built into the libreoffice package this will shave time off of system recompiles as it doesnt have to stop every second to start a new acct-group package and will cut down the total installed packages on the system. acct-user should be similar it's not like you can opt out to remove these packages anyways and they are usually crucial for the package to work.
Well, they are. We migrated to GLEP 81 2 years ago because it lets you predictably handle UIDs and GIDs easily. If anything, we should just work on making Portage handle parallelism better.
