Yes, you need better transit but part of better transit is accessibility and for many lower-income Torontonians, GO is a luxury they simply can't afford. If everyone is paying for it thru their taxes, then everyone should enjoy the benefits of it.
Also, by having GO are just part of the standard transit fare, operational costs can be reduced on the regular bus routes. Outside of local trips, most people travelling are going to a subway station. This is why so many routes start/terminate at subway stations and then head back to where they originated. If GO was an option for everyone many of those riders would go to the nearest GO station which maybe much closer than the local subway one especially in Toronto where the subway system is rather small. This means much shorter bus trips so that 19km bus ride is now, for example, only 5 km reducing operation costs and/or an ability to increase frequency.