If Carriers only appear in instances where members of the owning squadron are present (which is the most reasonable way of impelementing them), others won't see a ship "disappear" when it docks with its Carrier. If you're not in the instance, you won't see either the ship or the Carrier. If you are in the same instance as the ship, you will see the Carrier too. In Solo, you will never see anyone else's Carrier: in PG, you will only ever see Carriers owned by members of your PG. And as PG's will need to be catered for, Carriers will never be "Open only" (and as Solo players can create a PG of one, a ban on Solo would be pointless). In Open, you will only see the Carriers of players you are instanced with, not "hundreds", even at popular locations.
Adding UC to a Carrier isn't a big deal, as explorers can carry an unlimited amount of cartographic data anyhow, so they don't need to "run back to the Bubble" to sell it and free up space for more. Space is big, so the ability to cash in sooner rather than later will very rarely lead to loss of first-discovery tags. It provides a backup in case of ship destruction, and a way of seeing your progress rather than waiting ages to see your tags. It would also allow exporers to publicly report an interesting find almost immediately, rather than having to keep it a secret for days/weeks/months to avoid poaching of first-discovery tags.
The force-multiplier argument for BGS work is something I hope FDev will consider, if they try to impose a minimum squadron size for a Carrier. If Squad A is twice the size of Squad B, and A is entitled to a Carrier but B is not, and the Carrier makes the squadron twice as effective in a given operation, then suddenly A is four times as effective as B and gains an unfair advantage.