Genghis: My immediate reaction is to say you're wrong, but I think I know something of your background, which suggests you know a lot more than me. Therefore, I'm going to ask why.
Just to make sure we're on the same page, I'm not talking about abrupt, full scale deflections, nor deflecting multiple controls - I entirely understand how they may cause bits to fall off. However, according to my understanding the Vn/Vg/whatever you call it diagram gives an envelope within which you can fly, in terms of G loading, and speed (confined to the pitch plane, but rolling gliders is kinda tedious anyway
). Anywhere in there you should be fairly sure bits will not drop off?
<sidenote>I have however seen Vg diagrams that 'taper' towards the right, defining lower load limits at higher speed. That seems reasonably intuitive, and is generally placarded / noted.
Pace: I'm not sure as to your point - exceeding either limit could be by the tiniest percentage, or a large amount. Neither is recommended, the point that was made to me in training was that I have more 'room for error' in the loading sense than the velocity sense. To recap, the relevant certification requirements are:
1) That the aircraft is tested to Vd (max design speed), and VNE is set to 90% of that (so the prototype at least has been proved not to flutter/otherwise self destruct at 110% of VNE.
2) That the aircraft must sustain a loading of 1.5x the design limit (i.e. if it's placarded +6/-4, it must sustain +9/-6) without being compromised structurally for 3 seconds. Stuff may break, and you may need to throw it away afterwards, but it must remain flyable.
The sole point being, that you have more headroom in the loading dept than in the velocity dept. And frankly, at +9, you're going to really know you're pulling some.
Kinda sorry I chipped in now, but nevermind, I can always learn things. For the record, I've never gone over G or overspeed in a glider, or a powered a/c, and don't plan to!