Seems like a security issue doesn't it? But it's actually not. Emailing people in your circles won't actually allow you to obtain your circle contact's email address, but in fact, just their name will show in the send to, cc or bc boxes. Once you send a message to them, they can see your email address and if they respond you will be able to see theirs.
If you visit your Gmail settings page, you can actually set who can email you via Google+, namely, anyone, someone from your extended circles, your general circles or no one.
Is this a good idea?
I believe it's just fine. There really aren't any security issues with this new Gmail update that I can foresee just yet. The only thing anyone needs to be careful about is remembering that once you email someone in your circles, they can see your email address. For most of us, if we email someone, that's exactly what we want to happen.