Who's infrastructure do they use to inter-connect those 700 servers in 70 locations... I'm gonna bet on commodity internet.
I DO use VPN when I travel, but for securely connecting to specific end point networks, not more or less random other Internet locations.
Your phone is not less secure than your hard wired computer at home... If anything it is more secure since mobile networking is under such close scrutiny right now. Media and hackers just itching for something to embarrass one of the big cellular providers.
Regarding open WiFi and how terrible it is could be a long thread of debate on its own.