[B]anning sketching seems like an unfortunate policy. Museums should recognize the importance of sketching as a primary way that artists engage with the tradition. Rather than forbidding sketching altogether, it seems more reasonable to limit large drawing boards, easels, paints, sitting on the floor, or otherwise blocking visitors flowing through high-traffic exhibitions.
I think that sketching with pencil in a sketchbook 9 x 12 inches or smaller should be allowed anywhere. I'm not aware of any museum limiting note-taking with a pencil and a pad of paper. School kids routinely go through museums with clipboards. I see no reason to forbid sketching if it's done in neat, dry media in a hand-held pad.
Let's remember that many art museums began as extensions of art academies. Too many art museums these days think of themselves as extensions of the gift shop