As someone who occasionally is quite willing to pay $30+ for a Northeast Regional BC ticket between Baltimore and Washington instead of an $8 MARC ticket (and, heck, I have a monthly MARC pass paid for by my employer), maybe I'm not the one to comment on this. However, there are a number of reasons why someone might want to ride the Amtrak over the local commuter line when such alternatives are available:
1) Whatever the shortcomings of the Horizon coaches used on the Hiawatha, they're more roomy and more comfortable than the seats in the Metra gallery cars.
2) The Amtrak equipment has more space and more convenient space for luggage storage, I have ridden Metra with luggage, I mean it was OK, but I basically took up an extra seat. That was OK for the mid-day train I rode, but would be extremely annoying if one is travelling during normal commute times.
3) The Hiawatha is nonstop to Chicago Union Station from Glenview. The Metra train makes numerous local stops, and is probably a longer trip.
I'm a fairly regular rider (~2 times per week) of Northeast Regional 188 between Baltimore and Washington, and I'm not the only person getting off in Baltimore.