Hi Armiger,
As I said before, I find it very reasonable to see Dauntless, Siluria's and Endeavours to go Renegade (unpublished Battlefleet Koronus stuff supports this).
The 2010 rules provide in this. Thus Chaos does have access to light cruisers.
And, where there CL in the old times? With the Grand Scale of everything with the Imperial Army and all I can imagine that with the need of Conquest, not exploration, that no real CL existed that mattered to the warfleets.
Now, I do see why people do build them, because they can and it is fun to see how such a ship looks like. Should we provide rules for them? There isn't a strict line to say no, but there is also no reason why we should.
And with this we can appreciate Gothmog's Additional Ship Compendium. I think this is the best place for them. Just like ASC2.0 covers all holes within the Imperial Navy Battleship line up it can cover the Chaos Light Cruisers.
And lets not forget, in the end it is what you and your group/opponents decide to play with. Really, if all like them play with them, if not don't.
As for tournaments, they are rare. And Adepticon shows us that you just need to make clear what the organizer likes to be the rules available to the players.