I wonder...
If people can afford purchasing gear from Xûr on top of getting it through random drops why to hold back? Purchase that Truth or Suros Regime, upgrade it and hold on to it. Sooner or later you will have itch to use said weapons or situations that needs them / benefits from them.
"I can afford Truth, but I have Ghorn should I?" as question strikes to me as... odd because of this. Ghorn maybe is God tier PvE RPG, but it being solar damage means Truth would be superior choice to anything with void [shields or burn].
Can you afford to purchase / get weapon that is deemed to be good choice? If yes then grab it, some day you will want to use and you are going to be glad that you purchased it.