Even that would be better than nothing. I'd imagine that people would much prefer driving to Unionville than driving the whole way into Toronto.
It works if you drive to GO - which sometimes GO still feels like it serves, ignoring pedestrian and cycling access and transit connections - GO had a dumb campaign mirroring the Toronto Police telling pedestrians to wear high-visibility clothing to be safe. Probably because walking through parking lots are sometimes the only way to get to a GO station and they don't really care about making it safer themselves.
It's time to get past the drive to a GO station to take transit mentality.
I'm not crazy about forced bus-train connections on GO, especially when they're too short of a logical terminal point (which I believe is Mount Joy). They add a lot of time for those making the bus connection - GO schedules 15-20 minutes in many cases for connecting GO buses to arrive before the train departs. It's faster in most cases to stay on the bus to Union Station.
Unionville is in a crummy location - built to maximize parking availabilty and little else. The GO station used to be at Old Unionville, the historic Toronto and Nipissing/Midland/Grand Trunk/CN station until 1991.
I hated the minimal midday service on the Georgetown Line because the trains terminated at Bramalea - an even worse place than Unionville is these days and I came from Brampton. The 9:35 Brampton-Union express got to Union within 40-45 minutes; this ended up becoming a 1h10 - 1h20 minute trip with a forced connection at Bramalea (a connection that was not guaranteed; I was on a train-bus that was supposed to connect with the 10:15 train; the bus arrived late and the next train was at 12:15, the driver merely dumped us and said we were on our own.). I was happy when the trains were cancelled and bus service brough back. The schedule and timing was much better.
That said, I'm okay with Unionville and Bramalea (and Bronte, Appleby, and East Gwillimbury and all other seas of parking that happen to have train plaforms); you're going to have to accomodate auto commuters. It does mean though that the rail services also need to connect the urban centres properly - stations like Brampton, Markham, Allandale Waterfront, etc. that are designed more to be walkable/transit-friendly.