I wouldn't say having more or less Ability makes a Pokémon vulnerable or precious. Look at Delibird, who has three Abilities, namely Vital Spirit, Hustle and Insomnia as its Hidden Ability. All three of its Ability are undesirable.
While the Gastly line has only one Ability to date, Levitate (and Shadow Tag for all Mega Gengar). It doesn't make Gengar a bad Pokémon and he's even better as a Mega Gengar!
In my opinion, it really depends on the assigned Ability, rather than the amount of Abilities. :)
Edit: Then there are those Pokémon with great Ability but couldn't really make use its own Ability! For example Rough Skin was the Sharpedo line's Signature Ability, But the sea bully's relatively low Defense stat doesn't really help Sharpedo use its Ability to the fullest, while Garchomp, who later gained the same Ability as its Hidden one, excels with Rough Skin.