I suppose as Steam games tend to be cheaper than boxed versions anyway, they'd be shooting themselves in the foot by offering resale activations. But quite agree about the appeal of reselling boxed games.I agree, but it's still an onerous standard if you like the ability to sell used games. Some games I keep forever, like HL2, but here have been several occasions where I was so happy I could sell a title that I did not like. Some companies offer a set number of activations and even that is much better for resale, because it makes resale possible.
Also, most Mac games maintain close to their original value, so resale can be fairly profitable.
Just looking at my collection of 20+ OS X games now, I realize that if I sold the lot I could buy a gaming PC! Hmm.