@appleandcomb : I believe 吗 is necessary at the end of an interrogative sentence. But then I am not a native Chinese speaker.
There is no practical significance? (是没有实际意义的?) You mean the character 吗 can be left out? I've been studying Mandarin Chinese for only 7-8 months now. (Google helped me translate what you wrote above.)
I'm not 100% sure, but I think usually there is a 嗎 at the end of a question. The only time I have heard it dropped is when there's some element of surprise like, "YOU'RE A DOCTOR?!"
So basically, my explanation is the completely opposite of @appleandcomb. Please correct me if I am wrong.
嗎is not necessary, but if you skip it, it sounds very rude to my ears. And yes you're right, it can express surprise or suspicion.
like, you are a doctor o_O? You sure?