It's one of the remaining methods of not paying full price for a game that's legal.
I'm not convinced that paying £40-ish quid for a game that you complete in 20 hours and no point re-playing once completed is value for money.
If more games were like Mass Effect (86 hours to complete) with the attendant depth of story, characters & gameplay and production values then yes, that's worth forty quid.
So. Perhaps what the consumer should be asking is 'will you slash your prices by 75% (80/20 = 4) if second hand games are made illegal?'. No. Thought not.
That latest Splinter Cell game.. about 5 hours to complete. Utter rip off, played it and punted it on eBay to recoup losses, a tragic end (presumably) to a classic game series.
If your game takes 10 hours to play then it's worth a fiver based on Mass Effect 2's 86 hours gametime (not completed #3 yet but looking like 80 hours, too), cos if ME is worth £40 quid then _your_ game *isn't*.
I've nothing against game developers making money, many games provide loads of fun (Portal a great case in point) but be realistic about what it's worth FFS,