Standing makes for a much better atmosphere, particularly at home games. Means the singers don't get split up at all as they can stand together, when it's all seater reserved seats there's season ticket holders that don't sing in the midst of some of the singers, people not buying tickets at same time so they get split up anyway etc etc.
Away matches it's not so bad as the ones who don't sing don't seem to bother going to the away matches that often so always seems to be a better atmosphere.