My general position is this: I don't trust my phone provider. At all. Just a week or so ago there was an HN post demonstrating how an ad provider can get your full name, cellphone plan details etc just by calling an API from a page rendered on your phone. But I also don't really have a choice - AT&T or Verizon or T-Mobile, they're all different flavors of the same crap.
Do I trust my VPN provider unequivocally? No. But I trust them a hell of a lot more than my phone provider, and they can't sell my personal info against my browsing history because they don't have it.
A VPN isn't the answer to everything, but nor is it useless.