I think it really depends on local conditions. Around here, a lot of commuters and tranportational cyclists use heavier road bikes, like cross bikes, tourers, even heaver, older bikes. My son lives in a city in Canada where for, for reasons of climate, availability and road conditions, the mountain bike rules supreme. If you only have to carry food for yourself, a trailer may be overkill. For a family, it might be essential. If you travel on train or bus, a folder might be useful. Around here, the buses have racks, so I wouldn't need a folder.