Red Fire Dragon's range is like, one mile long (seriously, that thing just keeps going and going), so presumably in the stage you'll be able to get far away enough to cast it without being interupted. You also have all those meat shield decoys (IE, Siegs) running around to keep the monsters' attention off you while you cast.
Unfortunately, this will likely not be the case for PvP. Still, if you ever team PvP at all, it could be useful.
...honestly, IMHO you should put a point into red fire dragon just because it looks freakishly cool (well, that and it's very useful in the stage)
Currently I'm main-ing two full-time fisher-girls. They fish. A lot.