I feel like their niche would be to help deal with the 4 slot move syndrome, as one move gets additional coverage.If dual-type attacks work the way we think they do, then they're statistically worse than mono-type attacks.
For example, Flying Press would only deal neutral damage against most steel-types instead of SE damage.
The only way they would be good is if they bypass resistances like Freeze Dry, though that move isn't exactly a dual-type attack...
Man, this is getting so confusing.
I don't know if you can get a 4x SE or NVE or not though.
Zapdos resists both, while Sawsbuck is weak to both.