This was done so it was not necessary to run /usr/bin/updimg.sh everytime a simple change was needed, for example adding or modifying a script. It also allows the image to include NFS which everyone may not use. It would be a waste for that person to always have the /usr/lib/nfs binaries around, so to give back control to that person, they can simply add /usr/lib/nfs to /mnt/eon0/.remove and all those binaries will be removed at boot giving back the memory for other use. Similarly for other binaries (/usr/sfw/sbin/swat) and kernel drivers not applicable to your needs or hardware.
The thing to remember when making entries to /mnt/eon0/.exec is that it should be a non-interactive command. Commented entries are ignored. Exerpt listing of .exec
/usr/sbin/swap -a /dev/zvol/dsk/abyss/swapFor /mnt/eon0/.remove entries you can go wild and experiment because you are erasing it from RAM so if you remove something you realize you need, simply remove the entry from .remove, reboot and all should be back. I have included a default set and commented some that I have found safe to remove in some cases. Exerpt listing of .remove
/usr/sbin/ucodeadm -u /platform/i86pc/ucode/intel-ucode.txt
/usr/sbin/ndd -set /dev/tcp tcp_time_wait_interval 30000