I think you should not be charged for extra class slots, or at lease increase the classes to 10 like BO2 and have the option to purchase additional classes after that.
Yes, you have access to you other CaC slots on other squad members, but you cannot access them in-game can you ?
Nor will using a previously prestiged squad member level you up.
To have such a vast array of perks and attachments available you really do need more than 6 CaC slots to fully realise the potential for experimenting with different setups.
With such different maps and combat scenarios you require a varying class setup to counter different enemy classes/playstyles per map etc.