Why not; really? If you're in the PT for just the TH, put it in your main hand just in case; you're there to tag the monster and put your fabled TH on it anyways.
If you're actually doing damage, you should be using a better combo like Blau/Sirocco and swapping in the knife (and possibly hands) near the end of the fight anyways [examples that I can think of are Nyzul].
For soloing, who cares what they do.
The only situation I can think of is Dynamis when you're not pulling, in which case go with whatever the leader tells you to do (everyone has their own superstitions about it). And if you're not pulling, why are you there in the first place?
So; you ask why, I ask why not? And really, who cares?