Granted, something like CGC would probably be
better for cels or cards, because you don't need
to crack the damn shell open to appreciate them.
(Although I'm not really in favour of encasing
anything permantely in plastic, unless it's
mylar. I have non-mylar archival grade bags which
have turned cloudy over the years, and it's hard
to appreciate anything I have stored in them.
Anyway, according to a professional archivist,
you can expect plastic to last on average 50
years before it becomes brittle.)
As for cracking a CGC with a comic book in it,
the value on the sheet becomes void, because it
can't be guaranteed.