I've been car-free since 1999 and still haven't figured out why a single person with no kids or pets needs a trailer. If I had the need I'd own one. I just haven't had the need more than once or twice in 12 years.
The big reason I can see is bulk purchases. 20lbs of rice, bulk toilet paper and the like are more convenient with a trailer. But that's not a commuting issue, that's a car-free issue... and not all car free folks need trailers.
I borrow one on average about once a year.