Well - you've gotten me to bite.

From the PW-6 AFM:

AIR BRAKE OPERATION
With the air brakes extended fully, the sailplane gliding ratio at the approach speed is 6.4. Diving at 45° with air brakes extended fully does not exceed the airspeed of 141 kts. Extending and retracting the air brakes do not cause the sailplane to pitch up or down. Operation of the air brakes is allowed within the whole airspeed range. The brakes can be retracted at the airspeed below 89 kts.

NOTE:
When extending the air brake at the airspeed above 108 kts
the negative vertical acceleration is significant. Therefore the air brake should be extended gently. The aircrew should have the safety belts fastened tightly.

My thoughts - the full air brake usage on a normal landing, stabilized descent by 100', on speed is not going to be what "gets you". Just like in any aircraft if you throw in a configuration change close to the ground and don't compensate for it you'll pay a price down the line - too little and you'll float, too much and you'll land way short of where you want. There is a lot of elevator authority to arrest the descent rate even with the air brakes fully open.

Speed would be a factor - meaning if one (for some bizarre reason) flew the darn thing straight at the ground at high rate of descent and speed with the air brakes out (or in for that matter) if you started your round out and flare late you'd break people and glider.

I've landed the PW-6 numerous times and coached students through it. It does some things well on landing - the controls are very effective, the feedback is pretty excellent, the air brakes are very effective and easy to adjust in small increments resulting in being able to "dial it in" well. I like that the brake is a lever on the air brake handle (hate it when people land with the air brakes on all the way in the ASK21 and pull it back actuating the wheel brake smoking the tire on touchdown. Also the geometry between the nose wheel and the tailwheel allows more of a margin to help prevent a tail strike.

And as always, your results may vary.

Regards Tom