You should probably mess around a bit in Sanahtlig's damage calculator. For me, in my current setup, a weapon needs to have about 1400 extra base damage to beat a 3% Assault weapon of the desired element. That is impossible, especially considering how easily Assault weapons can be leveled to 6% or more.

But, I do remember there was a time when that number was way, way lower. And thus, using out of element weapons was actually feasible. In the long run, you'll purely be using on-element weapons, but as you are building up, temporarily you can use off-element weapons.