For those who are concerned about the privacy and are mindful enough to care about the privacy of others, putting any information on anyone else's computer (like Google's servers) is a leak. You don't know what is being done with those data, intentionally or not. And that would be the case regardless of whether or not the software is free/libre---it's being sent across a network to a destination you do not control.

Of course, information is leaked all of the time. Depending on the software that you use, your (generally, not you specifically) address book on your phone might be available to numerous remote services, and that is directly parsable by third parties, and directly tied to you and your contacts. It's up to you to consider your threat model. mynewtb's threat model is different than yours.