I'm sure I'm not the only one who doesn't the ads back after the maintenance is done.
I never thought I would have said that a tube station felt like the calmest place in the city, but mentally, it has an enormous calming effect to not be surrounded by messages.
I wonder if the "Journal of the Mental Environment" has documented the place???
I guess, being cynical, the first brand to pay for keeping the place brand free would get loads of great advertising/brand value (ahem, cough…) from doing so…
Petafloptimism 
I love some of the newer stations (Jubille Line extension?) that don't have ads anywhere. Or didn't last time I was in them, anyway. You're right, advertising-free stations are so calming, a haven, especially from the surface Oxford Circus.
I don't think the new King's Cross Metropolitan line station has any adverts at the moment either. It's also pleasingly quiet.
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