What difference does it make? It's not like they can't see the scratch/dent if I don't tell them.
It makes a difference. The last thing you need is them putting a note in their system that the customer admitted user damage. Depending on the Rep, they may try to deny you any support from here on out.
So, why did you call them? You can read the AppleCare policy on Apple's Web site and first hand accounts of what actually happens on these forums.
This isn't a new policy, and while AppleCare Reps may tell you or vaguely hint that your warranty is void --- it really isn't as there are consumer protection laws that address this issue. Basically, they have to prove the damage either caused the need for a repair or is preventing a repair from taking place. Most of the time, they just call to upsell you to a new tier of service and replace everything.
Apple repairs work in tiers (or at least used to). For example, your legitimate hard drive repair may be covered under service tier #2, but your slightly dented casing might be covered under tier #1. They'll offer to replace your casing for a set amount that's usually $$$.
I had to fight them over a 12" aluminum PowerBook that had minor dents in the bottom casing. Mail-in service (which are all contractors) didn't want to replace the dead hard drive even though the dents were caused years before the hard drive died.
After complaining to a higher level AppleCare rep, they replaced the drive and left the casing alone as it wasn't interfering with the repair in the first place.
A similar thing happened with my 13" plastic MacBook that had the case cracking issue (they initially blamed me, but ended up replacing the entire casing because it was a known problem).
By the way, if you do damage your Apple product to a point where repairing it (for a legitimate service issue) would be a problem, you can always cancel your AppleCare and get a refund.