On some airplanes, again, that may be true. On many, it's not. A glider, for example, will have substantial wing flex. I've flown on some airplanes in which it was very possible to walk out to the wingtip and bounce up and down like a diving board. Indeed, some wings flex considerably; movement of the opposite wingtip isn't necessarily an indicator of wing or spar integrity.
This is true of many light airplanes, as well as larger airplanes. You need to consider the specific airplane, it's construction, etc, when considering the question at hand.