I read somewhere that 形容動詞 are a special subclass of nouns. Contrarily 形容詞, the い adjectives, are actually a lot more similar to verbs. During the time that I have been learning Japanese language (almost a year) I have developed the notion that verb is an essential part of a complete sentence and な-adjective can not fullfill the duty of the verb. Yet, In The essence of sentence construction on Tae Kim's Blog, the part about Treating verb phrases as nouns, in order to make a sentence, な-adjective is used as a substitute for verb and だ comes after that only "If you don’t want to sound girly". In this example on Tae Kim's Blog
is being treated as a complete sentence and then we read:
Finally, let’s add a bit of motherly advice-sounding nuance to it and give it a more conversation style, since it sounds like the speaker is trying to admonish the listener.
If you don’t want to sound girly, you’ll want to add 「だ」 when using 「よ」 with nouns/na-adjectives.
My question is that, in order to make a complete sentence , 大切 being a な-adjective, why it is not necessary for a verb to come after it?