I don't see the big deal here. If you're signed in to your account, when you search using the search page it's integrating a search of your mailbox. The key here is that YOU will get results from YOUR account and you HAVE to be signed in to YOUR account to see it. Consider it saving you a step, with this integration you'll no longer need to actually go into your gmail to perform a search of your mailbox, you can just do it right from the main page.
It's not like Google is crawling your mailbox to add content to other people's results. They're not storing information from your mailbox in the regular search database, and the content is already stored on their servers anyway.