Years of Britney Spears Naked Screensavers and malware "removers" have taught us that users are idiots. You can net a bazillion users with one "unlimited space" add-on.
Obviously, there's some vetting but that doesn't always work.
These add-ons could run on Google's servers and only have connectivity to your inbox and session. Instead, this appears to allow companies to do what they like, sharting your data over multiple jurisdictions if they want. Doesn't fill me with confidence.