One could make an in-lore explanation of why both slash and direct-fire beams are present. For example, slash beams may be cheaper and easier to construct and maintain, have less space, energy and targeting requirements, they can be far less susceptible against EWAR attacks (in BP: WiH beam jamming doesn't affect slash beams in some cases), or they can be utilised to deplete large fields of active armor on enemy warships in order to soften them against subsequent slashes or bomber strikes...
Slash beams could even be just one of two modes (second being direct-fire) of a beam cannon, profiting from one of the above.
They are present in the game because a)
They look cool
b) they are a kinda big reason for the player not to get between fighting warships,
c) capship weapon balance - a certain warship might have a bit weaker beam cannon than other ones and player has to take it into consideration when prioritising his targets, but it would be hard to distinguish between 7 sizes of a beam, all the same color. When you see a slash beam you automatically think "It's weaker than the direct-fire beam, better take care of that other one first.". Right?