It is sort of related to the glossy vs. flat paint argument on restored warbirds as well - glossy paint will always hold up better, be easier to clean and will last longer in good shape, but of course flat finishes are generally more authentic. Some guys will only ever choose glossy paint to save their investment and look showroom quality, while other guys will only ever choose flat paint for authenticity/accuracy. There are groups/organizations that simply do not want to invest the time and energy (or cost) to maintain a bare metal aircraft, while there are others that do so with a great amount of pride. There have been times where with restored bombers, like Aluminum Overcast, FIFI, or even B-25's such as "Maid in the Shade" and "How Boot That", that due to the condition of the original metal skins (rash/pitting/outer layer of Alclad worn away/patches/etc.), they chose to paint the airframes silver for a nice uniform appearance. For an organization that constantly is flying their aircraft every day, it is also counterproductive to try and maintain a bare metal aircraft - the Collings Foundation had a real heck of a time trying to do so with just their TP-51C. Collings acquired the polished bare metal B-17G that used to be part of the Evergreen Air Museum, but before it ever tours with the Collings Foundation, they're also going to be painting it in an OD/grey paint scheme, so as to not have to worry about all of the time/energy required to keep the bare metal maintained. I know with a local B-25, "Miss Mitchell", when it was being restored back in the 80's, some of the guys involved wanted to simply paint it so as to not have to put in all of the work to polish the airframe, nor all of the work that would have to be required each year to maintain the finish. Fortunately, the voices of a few at the time prevailed, persuading the group that it would be a far better looking aircraft if it were bare metal (and painted in authentic, historically-accurate markings), and everyone involved with the aircraft seems to have agreed ever since that it was the right decision.
Expanding on Gecko's post - on P-51 production during WWII, from the earliest production examples, they filled the wing panel butt joints, screws and rivet heads, within the first 40% of the chord of the wing, and sanded it smooth, prior to the camouflage paint being applied. When camouflage paint was done away with in early 1944, the wings were painted in dull silver instead, so as to continue to protect the filler and to match the look of the rest of the bare metal airframe. Post-WWII, in the USAF, there were a great many P-51's (then called TF-51's) that were painted overall silver, by the early 1950's, after having gone through overhaul facilities and being rebuilt/zero-timed. Australia did the same with their Mustangs, painting them silver in the post-war years as well.
