I am not unfamiliar with Robert Spencer. Although he does cite authentic Islamic sources, he frequently relies on poor/inaccurate translations, or quotes them only partially or out of context. He does not typically quote from mainstream Islamic scholars, but spends more time reporting from the fringe and trying to pretend it is mainstream.
The attacks that Spencer makes against Islam are frequently based on weak information, and more often ignore the classical interpretations.
In other words, he deliberately distorts the truth to mislead the people about Islam.
So he is not an authority on Islam. His opinion is meaningless to Muslims. If you think one of his claims has validity, then bring the evidence, and I’m more than happy to discuss it. But his blogs, books, and websites do not count as evidence to me. If he’s telling the truth, then let’s look at the sources.
Edited to add: I’m not sure if it was Spencer or someone else, but Booklover and I had a go not long ago where she was trying to quote a source that doesn’t actually exist–something many of those who attack Islam try to do. It’s deceitful, in my opinion, to not list the real source, lest they be forced to admit its weakness.