Personally I wouldnt worry about de-humifiers unless you have something really worth while in there - a 250 Ferrari or D type. For a modern car with modern paints you are unlikely to have an issue but up to you.
Sorry but I must disagree, if you want to keep you car in as good as condition as when it goes in the garage - a dehumidifier is essential.
Without a dehumidifier my tools get condensation on, which in turn slowly rusts them - any paperwork left in their doesn't stay flat, and brake discs/nuts/bolts will slowly rust also.
Dehumidified everything in their stays perfect, which for a few pence of running costs after initial outlay is worth it imo. It also doesn't smell of damp, which is far more pleasant, and I can hang art work without fear of it being spoiled.
With regards to the car then if you have a concours/mint example it will stop chrome pitting, nuts and bolts rusting, and wiring from getting damp ingress.
For sure you could store a modern car outside in the rain with no issues, but it will be in better condition if you store it in a dry environment (RH approx 60%).
Really it all depends how clean and fussy you are with your car, and bare in mind I am talking longer term ownership prospects here, 10 years of one approach vs the other will yield significant differences.
Also I must say for many people a Morgan is their dream car, so it most certainly "worthwhile".
