Battalions can be useful if it helps to create features such as directional attack bonuses and formation bonuses.

Well, select a scenario map from the map selector and play it. Sorry, what am I missing?

Indeed pathfinding may be one of the main issues of bridges - walking on objects could be done with fields already (as they represent buildable objects) but finding a single entrance and exit while actually walking on water (denied area) may not be so simple for pathfinding. Again - in settlers 4 or 5, I remember you could build bridges only at pre-dertmined sites that neeeded definition in the map itself (defined entrance and exit points). Actually, should it be possible to build a bridge everwhere you want? This might be an interesting feature like blocking ships from entering or leaving a river or even denying access to a beach (if building a U-shape bridge)?. A bonus of freely positioned bridges could be that they could be possibly built on land as well (zero height above terrain) and would then be called "roads".   I guess that would be a completely different game then.

Thing is, I was never able to play a scenario - how is one supposed to do ? When I choose a scenario in the maps, nothing triggers.

So it should be a parameter somewhere, in order to make it work for everyone (those who like you want to control finely the repair actions, and those who'd prefer it to be more automatic).