Class 17 weapons are a catch-all for all air launched missiles. As you say - they are used firing at one particular target element in direct fire mode only, but due to the original legacy code design, must be HE warhead types. There is no differentiation between an LGB blast weapon, or a hellfire type HEAT weapon.
Air bombs drop bridges, however. Bigger being better in this case, as with arty over 8inch.
Then again - I can think of no game in the last few years where I actually wanted to drop a bridge, since most are in a position I would likely want to use later, tactically. In the defence I am happy to have them there on the enemy side of the map as a defile for him to funnel troops into, therefore a nice spot to blast with interdicting arty fires. Maybe drop a little wood bridge in his rear if it may cause some hassle - but 122mm arty does that job fine, no need to call the crabs in for that.
(Cratering his approach roads on a snow map with 122mm+ bricks is however a tactic I regularly use
)
Cheers
Andy