I disagree, and I feel like you're kind of simplifying something that shouldn't be simplified in the first place. Having dual-type moves would more or less significantly altar competitive play, as the types that were once known to be super-effective against others might not be super-effective anymore after gaining an additional type.
For example...Freeze Shock. An Ice move that had the potential to paralyze. What if it gained Electric has a second type, due to this? Now all of a sudden, you have a move that's completely walled by every single ground-type imaginable, and the move that was once considered super-effective now no longer has any effect whatsoever on the type that it's supposed to be super effective against.
And Scald. Would it have Water/Fire because of it's chance to burn? It's such a useful move in competitive play, but can Fire-types freely switch into scald without any fear of it causing super-effective damage? Would Heatran freely switch into Scald and get a free flash fire boost?
The point being from all of this is...if dual-type moves were to exist (and assuming that the aforementioned moves get an additional typing), it would make things unnecessarily complicated, and why? Why is there a necessity for dual-type moves, I don't see anything wrong with our moves being single-typed in any way, so why is it needed? Especially if future dual-type moves get introduced, what's going to happen then? It'll just be a mess of dual-type moves that aren't needed in the first place, nor do I think are they going to be needed anytime in the future. .___.;;