You are right; the goggled version brings up the 50mm frames and the goggles widen the veiw to match the wider lens. The non-goggled 35's will, as expected, bring up the 35mm frame in the M2, M4, M6 etc, and the guy in the store was blowing smoke.
Worse, the goggles change the focusing such that the lens can't focus correctly without the goggles.
Edit: Back to better again, I understand that the goggled 35 is as usable on other models as on the M3. That would certainly make the 35mm framing easier on my M2, where the 35 frame is almost invisble at the periphery of the viewfinder.