Well, I've said it before: there ain't NO way in Elmo I'm gonna give Hitler, uh, I mean Google my personal number... and I sure ain't going to give Yahoo! my number "for added security", when they don't even do anything as far as security is concerned in the first place, ala all the people who have had their accounts hacked in the past couple of years.
But yeah, like Drtooth said, after a while of being prompted, you COULDN'T avoid it any longer, you HAD to register a Google account to connect to your YouTube account, and like he also said, there's been some cases where people who didn't register a Google account lost all access to their YouTube account because of it.