I have Mint and live in a major city and have never had any noticeable issues. If you're talking about power users, i.e. people using their phones constantly on network to do a wide range of network intensive things like watching videos, then sure it makes sense to not use a MVNO. If you're the average person that spends 90% of their time on wifi and uses their network data occasionally when not at the home or office then MVNOs are a godsend.