I doubt the bike flexes more on the road than it does on a trainer. In many situations, the bike sways left/right as you're pedaling when on the road, where on a normal trainer it has no chance to sway. Your body weight and forces are spent flexing the frame rather than swaying it.
I'm sure going down a rough road or hitting bumps has its own set of extreme forces, but they aren't the same as the flexing you'll get on a trainer.
This is pretty much my thought as well. The swaying on the road isnt obstructed in any way. I´ll put my alu-bike in the trainer!