Mike:
I take your point, but a MATZ shouldn't really be an unknown environment. Military aircraft do need to operate above cloud, and will attempt to get VMC below in the safest manner possible. Even with only secondary radar working for us, we can get seperation on most other airspace users. With better airmanship from non squawking parties, we can get separation on all of them.
As a glider pilot myself, I know all too well that the best looking cumulus are always in the shark-infested custard airspace, and I have no objection to a MATZ being penetrated by civil aircraft, but a brief call to ATC can help increase safety for all parties.
As has been mentioned above, these kind of incidents do little to help gliding's case in the big mode S debate!