Gmail also drops sent mails occasionally for no apparent reason. They don't even bounce back, they simply disappear and never reach their destination.
I doubt it's malicious
I doubt it meets expectations
I think it meets profits.
I find it incredible that google can profit from Gmail. (Actually I find it incredible that Google can profit at all.)
Advertising is an incredibly profitable industry.
Gmail doesn't show ads anymore.
It does in the promotions tab.
It provides a pretty good incentive to keep that Google cookie in your browser though (as well as your access to your interests, online purchase history, address book etc. etc. ad nauseam)