We've asked the same question and our answer came through
Costello, our insurance carrier. Paraphrasing here,
'The instructor must on the field (on the ground) and
be in a position to observe the takeoff.' Being in
the towplane that is towing the student doesn't count.


At our club, the gliders have no radios, so the CFI
is really there for the launch and maybe tow (he can
still talk to the tow pilot on the radio).
This seems consistent with Costello and what our
insurance requires. Having the CFI ensure a preflight is
done, trim is set right for T/O, spoilers are
locked for T/O, tow rope isn't tangled, etc. can be done
on the ground right before tow.

Requiring a CFI for landing makes little sense to me
(other than for comic relief).