Just because you saw Radiance battleships supported by a rapture battle-cruiser in the rebellion trailer does not mean you should go off and start building that combination of caps. Theres no mistake a Rapture is a superb support capital ship and one of them does wonders for everyone of those radiance in terms of damage reduction.
Radiance's have low hull points so die easily without more defensive abilities. Unless they have some insane buffs otherwise:
it more scary to see Halcyons jump in with 1 rapture as support.
-long range (covered)
-fighter/bomber defense (covered)
-damage output increase fleet-wide (covered)
-damage reduction from enemy frigates and fighters (covered)
-damage output increased for bombers/fighters (covered)
-damage retaliation from vengeance for friendly capital ships, frigates and structures (covered)
-domination to steal overseers and hoshikos (covered)
All you need is a mother ship on top of that for shield regeneration and malice.