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.
I wonder about this, because one of my stiffest steel bikes steel bikes see some pretty hard side loads during hard accelerations and sprints. Those same forces would probably be quite harsh on a trainer where I couldn't rock the bike from side to side.