I'm just curious as to what's not "quite sensible" about traditional cards??
Unless it's postage? Which isn't relevant here, it's the same for a card or a letter, unless it's a very bulky card. Almost none of mine require extra post, the ones that do are usually so bulky I put them in with gifts.
Personally I love getting a real card that opens, and DH is even pickier and doesn't like the unusual formats that don't open in the old-fashioned way, but on the basis that any happy snail mail is better than none, I'm sure postcards would be welcome.