Just remember: if you delete a book that got to your Kindle while it was registered at the other account, you can't get it back. . .until you put it on that account again.
As to the ethics of it. . . .that's for you to decide. I, personally, would not be comfortable it, but I can accept that someone else might be able to ethically reach a different conclusion, so please understand I am TOTALLY NOT making any judgments here. I've actually had this discussion with my brother and we decided that if he gets a Kindle ('cause he'd get mine if I get a new one') we'll either share my account or have our separate accounts. If we share, we can share books. If not, he's going to have to buy his own copies. We do have an overlap in tastes. . .both like Grisham and those types of political/legal thriller books. . .but there's a wide divergence too. He's way more into horror and creepiness and I am much more likely to read other mystery, sci fi, fantasy, and historical fiction.
Ann
edit: Sorry, I should add that if we ultimately decide to share the account we'll have to put some policy in place so that he doesn't run up my bill.

I believe more than one CC can be entered so we could each buy with our own cards, or we could use a gift certificate system so that we don't go over budget. That is still to be worked out. . . .
Ann