"How hot are summers here?"
It also can be a question without "有". In my opinion, I think "有" in this sentence can emphasize how hot it is. Before you ask the question, you might know it's hot here, but you wanna make sure how hot it is.
這裡夏天多熱？ 這裡夏天有多熱？ are both correct in Taiwan. I'm not sure in China. Anyway, I don't think it would be a problem for native speaker.