Given that it's relatively easy to stop your opponent blowing a bridge (I've found) - your frustration is not surprising. A single mortar unit per spotted engineer unit near a bridge can usually prevent it being dropped by bombarding constantly as your assault troops approach. It's not 100%, but pretty high - maybe 90% success rate. I would think the key is to suppress whoever is suppressing your engineers, and if you haven't the units to do that then you will be in a situation pretty close to real life, maybe? What isn't close to real life is that you can't blow the bridge at will, with a simple command, hours before any enemy unit gets near, as part of your 'strategic planning'. But you can see why the present system was implemented, in game play terms...