Well, how many Acela customers are bringing more than a rollerbag and a backpack?
It depends on the day and train. For business-heavy trains, a fairly small number of people are traveling with full-sized luggage, but it's generally pretty rare. For my weekend trips, there always seems to be a couple people with outrageously sized luggage (granted, I'm the kind of person that will only take a carryon, regardless of trip duration).
I can only think of a couple of times with a couple oversized bags piled up in the handicap corner on the NER for a family traveling together
I'm a weekly rider (hooray for supercommutes!) between BOS and NYP, and there's always some larger luggage stored somewhere, but it's rarely a large amount, but still more common on weekends.
Regardless, I also hate getting nickled and dimed, but Acela is a premium product. If it's aimed at business travelers, then a big limit is less offensive since the target market generally isn't baggage heavy. It's also mitigated by the fact that there's a reasonably frequent and comparable service (NER) that won't have these restrictions.
All that said, it does seem to be a design failure not to be able to create a competitive advantage vs. airlines.