I'm not sure if it was so important for the RN, but for medium sized navies, merging roles to create a larger hull, can give the option to get over the critical size to operate F35 effectively (whatever that size is?). The JCI and to a degree the Cavour are examples.I'd say more dock & hangar, IIRC after the first 3 America class LHA's they're going to have docks due to past experiences about getting troops to shore.
The flipside is their aviation capacity will be sacrificed. Same scenario here IMO
The advantage is with multi role ships when money is tight they get more value out of the ships which helps getting them built. The downside is when you need them for both roles, you can't put F35 in the hanger if its full of tanks!