As Rainforest pointed out, there is tons of potential demand. In fact with fare integration, a line like this could turn our current transit paradigm on its head. It would truly become like a European (s-bahn type) system. Why would someone from Scarborough spend an hour taking a bus, the SRT, and two subways to get to Union when they could walk or bus to a station, take one train to Summerhill/North Toronto and subway to Union. One less transfer and 10-15 mins less travel time.
Like I said, GO is actually the missing link in regional transport planning. Transit City should have been planned around improving GO and integrating fares between GO and the TTC. Instead we get a Crosstown route that won't be any faster across town than anything already in service. It might actually be faster for some Eglinton residents (depending on how far they are going) to take the Bloor-Danforth line across town!