There's a wrong argument used by some people, not only against parallel worlds but also against any alternative activities that don't have to do with dm in LV, even if they are done in the main world. That wrong argument says that if we have people playing in other places and doing other things, there will be fewer dmers fighting in the main places. Well, in first place if those players are doing those other things, it's because they prefer to do them, so don't OBLIGE them to do what they don't want (if someone really likes dm, he will keep doing it anyway), and in second place if we offer more things obviously we will attract more players.
For example, there are people like enkei or like some members of the R-gang that almost never dm, are here only to drive/fly around, so if we removed races and Rustlers, they wouldn't dm anyway, they just wouldn't join the server anymore and it would be more empty.
Also, if we take that argument so strictly, duel arena and events should be removed too, because they take players from the main dm gamemode (duel arena even uses parallel worlds), but don't events increase the total number of players in the server when they are hosted?
So offering other things wouldn't reduce the number of dmers, but would increase the number of total players, and the proof is that there are other servers that offer alternative activities, some of them in parallel worlds, and have more players than LW.
The other argument used against parallel worlds is that they imply the presence of teleports, which according to their detractors, would be abused by players. Well, that really depends on the teleports: obviously if you can teleport from anywhere by just typing a command (like in some servers), then of course that you can easily abuse them, but if they are only possible from safe zones where you cannot be killed anyway (like banks, Ammus and the duel lobby), and after using them you come back to the main world at the exact same point where you left, then there's absolutely no possibility of abusing them.
And the perfect example of this is out current duel arena: it has parallel worlds and teleports, but there's no abuse in that.
So in conclusion, making some things in parallel worlds wouldn't reduce the number of dmers in the main world but would increase the total number of players in the server, and teleports can be implemented in a way that allows no abuse of them.