A project does not have to be located within an area to be seen as a benefit, and RER is one of them as it will reduce people using the subway, path will take people off the streets paving way for better streetcar speeds, etc. Every cause has an effect.
Also, who's to say no one downtown will use RER? I'd see it as a huge plus living so close to the terminal railway station, where its means I can get pretty much anywhere with ease. GO has plenty of new stations located in or near downtown that will be a huge benefit to all those living in the area. The presence of these lines also means quick trips to other cities (and most importantly, the airport) are possible without the need of a car.
Also, the transfer isn't the only benefit of a subway deeper into Scarborough, it's just the one that benefits commuters the most. I've already discussed most of these benefits in great detail (simplified operations being the biggest one). The state of good repair is in reference to any Scarborough transit project, the RT is old and outdated. It either needs to be renewed or replaced. Doing nothing is negligence, the thing I and pretty much every person on here argues against. There is also negligence with not addressing the crowding issues on the Yonge line, yes, but solutions are underway in numerous forms to mitigate issues (the biggest ones being the relief line and the discussion on Platform Edge Doors).
I rest my case...the discussion here is about the Scarborough subway extension and should not be a downtown vs suburbs debate or a priority in projects debate. If you want to do that, transfer it over to the Debate the Merits of the Scarborough Subway Extension Thread, because this is going nowhere.