On land, at least, more surface area means more rapid desiccation. I'm thinking of cyanobacteria around here, the terrestrial form. They're filamentous, one cell in diameter, but they're joined together with other filaments and held together by some kind of gelatinous material. In the summer they dry up and look like little pieces of scat, but after a rain they absorb water and look like seaweed. If they were mono-filamentous they'd have a hard time surviving even more so if they were unicellular.