Back when I was first pointing these out to be a completely new Covenant ship, rather than CCS Battlecruiser or even a scaled down version for the sake of video game trickery, I suggesting (apart from describing them as cruisers to be a massive mistake) that they would mostly likely be used as patrol boats.
http://en.wikipedia.org/wiki/Patrol_boats

Compared to real patrol boats, the CRS-Lightcruiser (urgh) still has a pretty big punch thanks to that ventral beam. Way above what the Lich would have, which is more comparable to a Scarab. That's what the Lich is, an airborne Scarab.