[r][ia] is only bad when for example you want to play tdm without [r][ia] but the only tdm with lots of players has [r][ia], so you make your own tdm without roomtags but no one/not enough people join so you cant play tdm with without [r][ia].
With duel this isn't a problem because you only need one person to join to play. Anyone can make a duel room without [r][ia] and someone will join. You will always be able to play duel without [r][ia] unlike tdm where you need lots of people to join.
