Because it gives them flexibility in what they can do, something which everyone for some crazy reason seems averse to on this board whenever something like this is suggested.
It's not just an 'anti-shadow' weapon. Fact is, we only really get a good look at the Vorlons in a fight once (Coriana 6 you never really get a good look at any ships - it's hectic), and it happens to be against the shadows. You really think that super-advanced shields that work against shadow molecular slicer beams would have any trouble working just as well against measly Minbari/Centauri/insert younger race name here weaponry?
It's not just an 'anti-shadow' weapon. Fact is, we only really get a good look at the Vorlons in a fight once (Coriana 6 you never really get a good look at any ships - it's hectic), and it happens to be against the shadows. You really think that super-advanced shields that work against shadow molecular slicer beams would have any trouble working just as well against measly Minbari/Centauri/insert younger race name here weaponry?