I am have a concrete pool installed and the builder attached a ground cable to the rebar before spraying the concrete. He told me it is up to us whether we want to ground it or not. Given the lights are 12V, I don't really see any point in this. Any thoughts on this?
I never knew that pool rebar was grounded or bonded, so I did a search and found what looks to be a pretty good article on the subject: http://www.poolspanews.com/2009/051/051grid.html
The lights in the pool may be 12V, but you've certainly got a transformer for those lights, a pump, and possibly other stuff in the pool area that is at line voltage, and you want all of these things and the pool itself to be bonded together and grounded.
YES YES YES... All metal parts should be grounded and bonded together.(to eliminate and voltage differential between separate metal parts) Just do it so you won't have "what if" thoughts every time you look at the ungrounded parts.
I have a bit of paranoia when it comes to electricity (took a few volts when I thought something WAS grounded but wasn't) so when it comes to things like that I always play it safe. If it's not going to cost you a ton of money, I'd do it just for safety sake. The lights may only be 12 volt but you never know in the future what you'd want to put around the pool or what might come in contact with it, better safe than sorry.