I have WiFi on when I am at home or work. When I am outdoors I often force it off because it ends up connecting to weak signals that are not actually useful and basically causes the phone to not 'see' any internet.
I hate this. there should be a feature that ignores weak signals beyond the whitelisted ones. whenever I turn off the wifi to ignore the weak signal, I wind up forgetting to turn it back on, racking up gigs of data in the process.
This has happened more times than I care to keep count. Everything considered crossing 2GB in a month is extremely normal for me