Having 3gb of data connectivity for when you're in the US is a pretty big upside for me, and whilst it may only be a $5 per month saving, the plan being free makes it far more of a "no brainer, no loss giving it a shot" option I think.
Also, it may just be sampling bias, but I'm not seeing anyone complain about VoWiFi and dual SIM voice over cellular not working on Helium. VoWiFi on Tello works perfectly well once you figure out the address set up requirement, but connectivity on cellular seems a bit more patchy?