I'll put myself firmly in the one-device per device school.  I can tolerate
the minor numbers/density hacks for 1/2 inch tape, though it's ugly.  However,
there is *no* reason for the kernel to take over this monitoring.  Most
visibly, there are cases where it will be wrong.  You can decide to ignore
a lock file.  You can wilfully remove it.  But if ever, ever, there is a bug
in the kernel's code for device locking, you can have bigger problems.  The
kernel should not be the one with state awareness for that device.  All user
programs can agree to be responsible about checking their lock files, and
programs that have good reason not to pay attention (stty comes to mind) can
do what they want.  (stty?  Yes, stty; haven't you ever gotten a voice call
from someone asking you to set a bit on /dev/ttyxx because they can't type
until it's set, and etc. etc. etc.?)  But if the kernel thinks a device is
locked, Bad Things Happen.

-s