Is it just me, or does the CPV have absolutely no use in mid game? Most people (including myself) hardly build any of them, as we only research them as a requirement towards the sweet, sweet CCS. They take basically as much time to build, and don't even have an energy projector. In short, they are under-powered and not worth it. It would be nice for them to actually have a purpose mid to late-game. Perhaps they could utilize the ion cannon (pink mist from haven star bases) as a "stepping stone" towards the energy projector as the covenant discover more Forerunner goodies.
Also, "The CAR Covenant Frigate is the smallest ship-of-the-line boasting defensive shields and much needed fleet support." -http://sotp.wikia.com/wiki/CAR-class
Wait... there's a ship SMALLER than a CRS!? It would be funny to see this in game compared w/ Seraphs.