Pure-Types vs. Dual-Types | Bulbagarden
Anyway, yeah, dual-types are not all that fantastic, but they usually are. For example, Normal/Ghost is better than both pure Normal and pure Ghost because it has more resistances than either one of them, and less weaknesses than Ghost. As attacking types, both are not bad, so the Pokemon can choose to go with one of the STABs if it chooses to (usually not both, though). Also, another example is Grass/Poison. Grass offers a Ground resistance that helps Poison, and Poison relieves Grass of two weaknesses, so as a whole, the combination is great because the resistances are pretty good, despite some weaknesses. Ice/Rock has a lot of weaknesses, but at least they have fantastic STAB, so if it's an offensive Pokemon with high Attack and Speed, then yes, the dual-type comes in handy.
Of course, sometimes stats can play a part of this, but usually an offensive Pokemon prefers two STABs, like Infernape and Kingdra, because they provide more offensive power. Defensive Pokemon prefers the unique resistances like Skarmory and Ferrothorn that can only be afforded by being a dual-type.
However, when comparing certain Pokemon like Pinsir & Heracross, it seems that the dual-type counterpart will be destined to be the more useful one. Their stats are not exactly the same, true, but they are similar that they can perform. In the end, Heracross is the more useful one because it has a dual-type.
I like it when a pure type Pokemon has a good movepool because I feel that they got some attention. Dual-types usually have more movepool privileges, like Jellicent: has the moves that Water-types and Ghost-types usually learn. There are some pure types that stands out, like Pinsir, who despite being a Bug-type, learns moves that Fighting-types usually learn like Close Combat and Stone Edge. Tangrowth is another example, because unlike most Grass-types, it has a large and great movepool that consists of good offensive and support moves.
That's it for now.
