I have said for years that it's better to use your own data connection, but not everyone has a lot of data or the ability to tether if wanting to connect a tablet/laptop etc.
A good Wi-Fi service that has an external antenna and a fallback connection to a second network (or potentially even more than two networks), including access to all bands in use (some 4G phones people use don't offer access to every frequency, which can limit coverage) and the fastest modem (again, many phones may not be equipped with multiple antennas and the latest encoding), is increasingly becoming workable.
Back in 2006 I had a meeting at Hertford House with the MD of FCC (Elaine Holt) to say that I didn't think Wi-Fi on trains was a good move, and we'd all be better off using our phone data. I could have been partly responsible for it not becoming something that was rolled out quickly as a result!
Well, in 2017, Wi-Fi has improved with 5GHz access points having less congestion and Icomera and others offering very good data management.
As such, I've been very impressed with it on GN trains, and VTEC, and would now feel much more comfortable using Wi-Fi than before. Of course, the improvements in 4G coverage has played a huge part here, and coverage and capacity is improving all the time.
However, I enjoy using Wi-Fi when it's free. I am not sure I'd pay money for it, as then it makes more sense to use my own data even if I could find my phone losing connection before the roof-mounted, external, antenna does.