Well CSM aren't cheap if you want a heavy or additional special weapon.
10 CSM, Champ w/fist, 2X melta, IoCG, rhino = 255 (I4, re-roll failed morale)
7 PM, Champ w/fist, 2X melta, Rhino = 256 (fearless, feel no pain, blight grenades, MoN)
At 10 man, you can't add an IC, while fitting in a rhino. Past 10, there is really no incentive to add models because you could just make a second squad and get more special weapons.
I could see the use of having a large squad of them running behind a rhino wall to disrupt deepstrikers/ counterassault.