I'm a regulatory lawyer working in telecom and commercial space policy, though I'm not sure it's my practice area that's primarily driving my thinking here. (Though it may also be true that, since I am not primarily a litigator, I have different confidentiality concerns in mind that some other lawyers. My firm and I do take these issues very very very seriously--perhaps in ways that would be impractical for a different firm in a different area of practice.)
Although I'm certainly aware of the proliferation of outsourced doc review and e-discovery, I would hope that the firms using these services are reviewing their agreements with these providers to make absolutely sure that they do not include language like GP posited: that they may "keep a copy of it and sell the contents to third parties or use it for advertising." Whatever the implications may be for attorney-client privilege, this also creates a risk of actual disclosure to hostile third parties, which would be a huge issue if you were working on anything commercially sensitive.
I'd also note that, depending on one's practice, you might also find yourself adverse to national governments. That creates a whole new risk profile with respect to could-hosted document management services, etc.