As far as myself: for comfort, when flying medium or long haul international. Most airlines, at least in premium cabins, provide socks as part of the ammenity kit, on longer-haul journeys. If I am not in a premium cabin, or on a carrier which does not provide such kits in a lower class of service, I usually have an (unused) extra pair from a previous trip or bring an extra pair of comfortable socks with me to wear in flight (primarily when sleeping).

Keep in mind I'm not talking about short US Domestic flights like ORD-BUF, ORD-LAX, etc, but long haul international flights which last in duration between 7 upto 14 1/2 hours (or even slightly longer) depending on where I am going. If I'm on a 9-10 hour flight and want to sleep 6-7 hours, it's much more comfortable to do so without your shoes (especially when flying up front in the big lazy boy). On longer flights, in addition to the ammenity packs with socks, some airlines will provide pajama's to passengers as well in their premium cabins.

However, if getting up to use the lav, I do put my shoes back on -- but even on such flights, you'd be surprised at how many people don't.

There are flights and times when it's appropriate to remove shoes, such as long haul internationals, and flights and times when it is not appropriate (i.e. run of the mill US domestic flights), IMHO.

Best,

SDF_Traveler