Ah, most people seem to dislike the toads. That or they prefer the Mario Bros. more. I think my younger siblings and I are the only ones that use Mario and Yellow/Blue Toad duo team ups when we play. It's mostly because my 3rd youngest brother has always had the role of Luigi. Even during our childhood, I'd always be Mario and he'd always be Luigi. It's carried on to practically every game from Mario Kart, to Mario Party, to Smash Bros. My 3rd youngest brother doesn't really play NSMB Wii as much as my siblings that are younger than him, so to not mess up with his powered up Luigi or amount of lives (and the fact that my younger siblings have "assigned themselves" to certain characters) they play as the Toads instead of Luigi. There's exceptions every now and then like when Luigi needs to be re-powered up, needs to get his lives back to 99, or when my 3rd youngest brother is playing as Mario instead of me.
Out of curiousity, when you play NSMBW with your wife, does your team consist of Mario and Luigi, Mario and Yellow Toad, or Mario and Blue Toad? It's just that I think it's, insanely odd, for a guy and a girl to play as Mario and Luigi. I mean, I've always felt a Mario and Luigi team up should be between two brothers or two friends that are guys. Toads seem like more a character girls would use, seeing how girls like cute things.