Trailers are extremely useful items when you need them. When you don't, which for most of us 99% of the time, they can be a storage nightmare... after all, how many urban dwellers have a spare 5 x 8 ...
Bike trailers are super-handy if you need to haul cargo, but they can become a burden the minute you empty that cargo and just want to ride your bicycle without a trailer rattling around behind you.