Yes, Psychic has generally been a bad typing as an attack, but defensively as a typing it has its fair share of usefullness. As its counter part, Ghost has actually been a much better typing than Psychic type because Ghost is immune to Normal and Fighting types, and unlike the Psychic who only resists Fighting and not being immune. Psychic does neutral to Ghost and Psychic does not very effective to another Psychic. Now comparing with Ghost type, Ghost hit super effective to both Psychic and Ghost types. In term of coverage and defensive capabilities, Ghost has usually been a superior typing than the Psychic. Game Freak were just having too much fetish with Psychic types for weird (pedophilic?) reasons we may never know.So Psychic generally isn't a good type? What about Ghost? Does Ghost give good coverage?
If Latias wishes to had become a Dragon/Ghost type instead, she would done so immediately. Think of Giratina, it has always been a frightening fiend mainly because of its Ghost typing, and of course, as well as its humongous stats.