And I'm actually fine with single types. Some dual types just suck so we're better off with some single types. *glares at Normal/Flying*
Sorry I meant Ice/Poison. I know what you're saying but dual types allow for more combinations in a 6 party team. It's more useful because of the extra coverage and the extra STAB. It would just be so fun to use a Water/Fire as well, right? Doesn't anyone agree?
Lang Zi says 'Furio Tigre can only be half the beastly hot man my descendant can be...' You go Shifu!! *Fangirl Roar*
Not all Pokémon benefit from being dual-typed, because chances are that the two types don't guarantee perfect coverage or a good defensive typing. Dragons achieve very good coverage with Dragon typed STAB plus something to hit Steels (usually Earthquake, better if STAB), but being dual-typed with Flying or something (Bug, for example) doesn't really help them, especially with that x4 Ice weakness. Water-types are similar, as are Fighting-types, among others. Defensive Pokémon might only use one of their STABs and leave the rest for support, so the need for two STAB moves isn't really there.
If you mean from a flavour perspective, single-type Pokémon are good at demonstrating the variety of traits that comprise their type, and these will likely be revisited in future dual-type combos. Mienshao (rather graceful ferret-thing) is quite different from Conkeldurr (construction worker), who is in turn quite different from Throh (judoka) - these all representing different styles of fighting. And so on and so forth.