I was there this summer - spent a few nights in Glenfinnan, which I can highly recommend -- but it was clear that most people were indeed driving through, hoping to stop for a little while to see the steam train (which, as it was summer, at least it was running!) and then going on. People were parked all over the place, obviously in not real spaces, and I was happy I was just strolling up from my hotel, but I did worry about anyone else getting out. Then as I went to the "easy" viewing space for the train (we had hiked up the big hill and around a couple of days before, which was majestic), it seemed I was surrounded by people who had been dropped off by a member of their party because they were STILL searching for parking -- now they were calling to say there was nowhere to leave the car so they wouldn't make the viewing "party" for the train's passage. It definitely is a mess.
My favorite way I saw the steam train (besides being on it!) was from a cruise out on the lake on Loch Shiel Cruises. they time a morning cruise so they pick you up and basically position the boat at the head of the loch to watch for the train coming "round the bend." It's a wonderful way to see it and helps you understand the topography of the line in that section.
Yes, I can imagine if one were trying to live or work there, this would be VERY worrying, as people simply aren't being considerate at all, and it's a small place, easily overwhelmed.