Dina is mentioned only several times in the Torah - her birth, her rape and the massacre, and her counted with the 70 souls coming down to Egypt. The midrash tells us how Jacob mistreat her with Esau and that subsequently, Simeon married her.
What is the necessity to include her in the Biblical narrative?
A midrash, for example, alludes to a daughter of Abraham too, but she's not mentioned in the text. On the other hand, as a sister of the 12 tribes, she has no special merits, there's no interaction between her and her brothers or parents, she's totally objectified and leaves no distinctive progeny.