The indefinite article certainly adds something, creating a slightly different shade of meaning, but is there a clearly defined rule or principle for this?
What kind of freak show is this?
What kind of a freak show is this?
Or:
What kind of person are you?
What kind of a person are you?
There is a TV show in which a character, unsatisfied with the manner in which he's about to be killed, asks rhetorically:
CHARACTER: What kind of a death is this?
What would change if he said instead:
CHARACTER: What kind of death is this?
Both are grammatically correct, I suspect. Ngram says so, anyway.