Usually if someone is going to demand a DPS number be given, then it's always important to give the context of that DPS number.
"I do 3.5k DPS" is a useless statement. For all the reasons stated in this thread. 3.5k on a heroic dummy? A critter? Patchwerk? Anub'arak?
So, saying something like "I do 3.5k DPS on a target dummy" is now more clear. I have context. I can at least compare that to my own experiences or the experiences I'm familiar with of people in the same class as you. However, some classes scale differently with buffs so this can be an unreliable number.
"I do 3.5k DPS in 25 man on Patchwerk (or Koralon or Anub'arak or Twin Val'kyr or Ignis)." Suddenly now I have a better idea what it means to have you in my group. I can tell your performance in a relevant fight with full buffs so that I can compare it to my expectations.
Lastly, if you have to give a DPS number either for a PuG or for a guild application, make sure you pick a fight flattering but meaningful. XT is certainly flattering to melee because of the ability to pad the meters during heart phases, but it's not giving someone an accurate picture of your DPS. Ignis is also a mixed bag. It's a great fight to benchmark melee DPS but not caster DPS because of the frequent pauses in combat due to Flame Jets.
So, as so many others have said, the answer is "it depends." If you have a rough idea what your DPS has been on the fight in question (for example, if you're trying to get into VoA and you know your previous Koralon DPS) then give that. But ultimately tell the person, "I have the right gear for this fight, I've done the fight before, and I do very good DPS. If you don't like my performance, feel free to remove me at any time."