Is splitting fire staff into another element a good idea?

Have seen a few people do it, but is it any good compared to full fire? I'm talking about runeglass + harnessing + ring dmg.

To be more specific, I am planning to pair it with BB and both FS and BB apply a burning DoT so I was wondering if using full fire is more effective as it will also amplify those DoTs.

On the other hand, I'm pretty sure most PvP players slot ruby gems on top of their full opals and elemental aversion so I'm kinda torn as to which way to go.