I think I found out the reason why musoft's partition boot sector reset
the floppy disk.  It is because they modify the diskette parameter table
pointed to by int 0x1e, and they need to reset the drive so that the changes
can take effect.

>From the comments people made, it seems we inherited it by osmosis.

It is no longer needed, as we don't modify the int 0x1e pointer to the
diskette parameter table (although, for maximum flexibility in booting
from floppies, we should be doing that rather ugly mangling -- unfortunately
it tends to cramp your boot sector a bit).

Oh yeah, I think I accidentally posted my new start.S & disassembly
to current users instead of here, but it's gone from the mail queue
so I can't tell for sure.  Oops.