First of all, you have to define "benefit". If by "benefit" you mean shaving 5 minutes off of what used to be a 40 min bus trip, is that really a big deal? Say you live at Sheppard and Kennedy, would it be a greater benefit to have an electrified Stouffville line running 15 minute frequencies into downtown, or to have to travel by Sheppard East LRT, Sheppard Subway, and Yonge Subway to reach downtown. In that case, the amount of time shaved off that trip would be nearly half.
The other thing that electrified GO with additional stations would do would be to increase the turnover on existing bus routes. Take Eglinton for example. The current Eglinton West bus needs to run the nearly the entire length of Eglinton West before it reaches a rapid transit route to transfer onto to go southbound. This means that there is very little turnover, except at Eglinton West and Eglinton stations. With electrified GO, you would be adding in 2 additional transfer points, and there would be a lot of turnover at each of those points, freeing up space on the buses.
Same thing goes for routes like Finch West (Barrie line), Eglinton East (Richmond Hill line and Lakeshore East line).