I am off to the Costa Blanca in a few weeks and can't decide on whether or not to take a 50m half rope in addition to my 70m single. Is the additional faff and luggage weight worth it or can you get by fairly well on 35m abseils?
Depends what you think you'll climb. For what it's worth, I've taken a skinny 50 as well as a 70 single on a couple of CB trips, and never actually used the 50. However, I go there primarily for convenient, sunny, single pitch sport. If you think you may get into bigger stuff/trad then I'd take the extra rope.
By the way, I wouldn't try taking the skinny in your hand luggage. They may well accuse you of planning to tie up the cabin crew!
In the past, going there as a pair, I've always managed to get two half ropes and a sport rope and a full rack between us without exceeding the Easyjet baggage allowance (Ryanair would be a different matter) - simply because, as you're not camping, there is only your climbing gear and clothes to take (and you won't need warm clothes at the end of April!). It would be different if you're going out on your own and meeting people out there of course.
It simply depends what you want to do - I've always wanted to do multi-pitch trad as well as sport, so I've always taken the gear, and I don't think I'd want to do something on the Puig Campana or the Penon with only a single rope, if for no other reason than ease of getting off if it goes pear-shaped.