I drove my last Class C without dash A/C for 15 years (it went out shortly after buying it, never bothered with the $$$ to fix it and actually ended up gutting it when I needed to work on the steering system and couldn't get sufficient access). I simply ran the generator and roof air when it was hot.
But...I just purchased my first Class A (with a working dash A/C), and I now realize that I should've fixed it (on the Class C) when it went out. Yes, the roof air helped, but only made the cab "bearable", not "comfortable", while driving across the desert in 120 degree heat. I just took my first trip with the new Class A, and it was REALLY nice having the cab area actually comfortable/cool while driving.
So my vote? Fix it; you may not be "glad" you did...but you WILL be "sorry" you didn't.