Upgrading the weapon makes it more powerful. It's why legendaries and exotics have a range of damage they can do. While your perks will go away, the total damage will be completely intact. So you level your exotic from 260 to 300, the exotic will stay at 300 making your grind completely relevant, the only difference being your weapon power will now be increased and your perks will have to re-earned in order to max out the weapon to the new limit.
So yes, continue to upgrade your weapon. It's not pointless and will save you time when the new content is released.
I was under the impression that the weapon doesn't need to be maxed out before being reset. If that's the case, why should he keep wasting materials and xp on it? Just wait for the 9th, cut his losses and upgrade it then. It will jump from 274 to 300 straight away. This system isn't that shitty if you take weapons you didn't level before (well, apart from creating 2 more bottlenecks where none was needed: glimmer and strange coins). But if you have a fully maxed weapon it feels like a waste. Certain weapons like TLW, for instance, only begin to shine when you reach the exclusive upgrade. Giving that up, and all that time to get it there is annoying and frustrating. They want us to redo everything again basically. People are pissed and rightfully so. There are better ways to implement something like this and they chose the worst they could come up with, just to pad the game even more.
Give us another column, make it "absorb" double the normal xp to be filled, if you must. Make it triple XP, I don't care.
I think the reasoning behind this is to prevent people from making the new Raid easier by buffing their weapons to 330, no strings attached. By implementing this, the way they're doing, they're making people say "fuck it, not gonna reset my weapon" and play the raid for the new weapons. I get why they want that, it makes sense.
But it never works. People have been cheesing the game since it came out 3 months ago: loot cave#1, loot cave#2, templar and atheon cheese#1, atheon cheese#2, everysingle nightfall has cheese of some kind, enough that all of them are easily done soloing with the right elements. People will find a way to cheese through the new content. Blocking our progress just to "guide" us towards unlocking the new, "proper" weapons isn't acceptable, not the way they want to implement it.
Like I said, I understand why they're making us "sacrifice" all the hardwork. Earning 330 weapons shouldn't be all easy peasy, you have to earn them. But I think adding another column, with more xp than usual (tie it to a special token obtained in the raid, if you must) would be fairer. It would make people want to play the raid not just to get new toys, but to get the means to improve their favorite weapons. My favorite is TLW. I doubt there will be another gun like it in the expansion. So I wanna keep it. Do I want to grind 24/7 like a madman just to keep it relevant? Not like this, no.
I really like this game, even though it has some serious flaws, but this is just asinine reasoning by Bungie.