Propeller Toad
Dislikes the amount of neglect Toad receives...
You're only considering games where the character is not being ridden, and even then you only talk about the Super Mario Bros. series. Games where Yoshi can be ridden count as games that Yoshi is playable because the character riding Yoshi does virtually nothing when on him. By this, Yoshi has actually been playable in many more of the mainstream games than Toad or his species has (Wario was actually playable in Super Mario Land 3, making him just as playable as Toad's species). Also, Toad only gives out items that can be found anywhere else (other than the Anchor and the Warp Whistle, which are optional), making him more of a convenience rather than a necessity in games like Super Mario Bros. 3.
I will not deny Toad's importance in the spin-off games, however. He's hosted a couple of the Mario Party games and has probably appeared in every spin-off Mario has. He also is the first supporting character in the series. But I consider mainstream Mario games before spin-offs, so I think of Yoshi as a more important character. If you consider spin-off games to be just as important as mainstream ones, then Toad can be considered important enough to have a game.
I will not disagree with your point about Yoshi being more important than Toad in the main series. However, it just bugs me when people say that Toad is not important overall to the series. He is pretty important as a character. While his importance in the main series seems to be like a teeter totter (ups and downs in importance), you do have a point about his importance to the spin-offs. Regardless, Toad is still an important character who should at least get one new game.