As far as I'm concerned, the entire point of using gift bags in the first place is that they are reusable!
That's why I don't write directly on them, and I tend to go for pretty colors, patterns, and designs rather than something with a printed message. And I will totally reuse the tissue paper inside too, as long as it's not torn.
POD. My family is very conscious of collecting and reusing gift bags. For me a bigger etiquette question is, does the gift bag now belong to the gift recipient, or should it be handed back to the gift giver? On the surface, as part of the gift, it belongs to the gift recipient; but my mom didn't want to accumulate gift bags so she always pushed them back on the giver.
We are careful with the tissue paper, too (even if we bend or crinkle it on purpose) and often reuse it as well. Of course it doesn't last as long as the bag but you can usually get two or three uses out of the thin, cheap stuff. We also collect and reuse bows and ribbon as long as they look good, but actual wrapping paper tends to get thrown away. I think the wrapping paper only looks good when it's very crisp and fresh (I don't even like transporting wrapped gifts somewhere because the paper gets dinged) whereas the other things can be "gently used" and still look fine to me.
I like to think we aren't crazy about it--we don't freak out or guilt someone if a bow gets thrown away, for example. We also have mainly adults opening gifts, though, who are generally more careful with the resuable components.