The raiders did seem a plot device to get us to this point. Or were they and Lord Kiro already being mmoved by the shadows? Since the raider ship was blown up, was that all of them? I don't think we see them again, do we?

At this point I think Morden & Co. just take advantage of events with the raiders to impress Londo. The rest of that arc is about increasing Centauri dissatisfaction with the way the Republic is going and the lengths some nobles are prepared to go to (and the risks they'll take) to change it.

As far as the raiders go, Garibaldi mentions that they'll take a while to buy fighters and recruit pilots, and with their carrier gone, and their prize too, they're out of business permanently.