The A380 had no immediate competition when launched, so adding winglets at the time was unnecessary. Now there are plenty around the airlines, Airbus can use the winglet as a way for airlines to say 'We've got the latest A380s, not old ones like those over there.' Hence sales of the latest model to those customers who want the most up to date looking fleet.
With regard to the P-8 Poseidon, it is a maritime patrol aircraft, which on occasions will rely on Mk1 eyeball to look for people and objects lost, or not wanting to be found, at sea. Putting a great big view blocker out on the tips, behind which things might get hidden and which also hinders the focus of said mk1 eyeball, wouldn't make sense.