Each implementation that exhibits different behavior doubles the amount of testing needed to ensure cross-platform correctness.

Not really. The whole point of undefined behavior is that the standard intentionally leaves out any definition of behavior, or the program is already fundamentally broken.

If you insist on unwittingly using code that relies on undefined behavior, you're the one mindlessly writing broken code. It's not the standard's fault that you're adding bugs.

The irony is that compiler implementations also leverage undefined behavior to save you from yourself and prevent programs to crash when they stumble upon the mess you've done with the code.