Though my top 3 favorite pokemon are dual-typed, Weavile, Swampert, and Yanmega, the following two in my top 5 are single typed, Ninetales and Luxray.
Whether a pokemon is single or dual typed doesn't necessarily matter to me.
Where I like the pure Water Feraligatr, I also like the Water/Flying Mantine, Where I like the pure Fire Darmanitan, I also like the Fire/Fighting Infernape. Where I like the pure Fighting Conkeldurr, I also like the Bug/Fighting Heracross.
Being a single type pokemon simply means they don't have a dual-type. It doesn't mean they're any more or less of a pokemon or team member.
Going down to the pure numbers and formulas of the game, even then, simply being dual or single typed doesn't hurt or help a pokemon. It's truthfully how their chosen typing works with their movepool and stats.
The Pure Normal type Blissey is an amazing special wall thanks to it's immense bulk and it's -lack- of weaknesses in being normal typed. The Grass/Steel Ferrothorn is just as impressive of a wall with his myriad of resistances, his impressive Iron Thorns ability, and supportive movepool nets him a valuable spot on most teams. The Pure Fire Darmanitan can cause devastation to most unprepared teams with it's obscene offensive fury given by a 140 Attack stat, Sheer Force, and high power moves, and fair 90 speed stat. The Dragon/Ground Garchomp was even banned from competitive play for it's ridiculous power, being strong enough to guarantee at least one KO if played right, thanks to his obscene offensive STAB, strong 130 attack stat, solid defenses, and the unforgettable Base 102 Speed Stat.
o3o; tl;dr: -It doesn't matter if a pokemon is dual or single typed, you can like either without a problem, and using singles or duals in trained teams is negligible, because a pokemon's strength is not weighed on just it's typing.