Used games are a buying and selling of your property. Restricting used games are restricting what you do with your property, within the basic property laws. That's... sort of something to stand up for.
As for the money side of it- to some gamers, that slight price difference can be the difference between buying the game or not. Or, perhaps, (as has been the case with me), buying the game without DLC, or buying the game AND DLC.
It's also a way to know that, if you spend money on a new game and find that it's either really bad, or really doesn't appeal to you personally, you can then trade it in. Of course, you can't get a refund- but you can get some money back, or better yet, more credit than cash towards a new game. Again, some gamers can't afford to buy a game new if they're not absolutely sure they will either like it, or they can get some of their money back if they don't. (And, once more, I've been there.)
I probably wouldn't be a gamer if it wasn't for used games, used consoles, used controllers. And I make a point of paying the developers back by being very generous with my assumptions on their DLC- which, it's worth noting, never goes down in price as the game does. I bought Dragon Age 2 for pittance last year. After discovering I liked it (despite it's flaws), I bought most of it's DLC- most of which cost more alone than the game did. So not only am I able to keep gaming- I'm still doing what I can to support the people making these games. Take that how you wish.