In my opinion, bias is bad when it comes to using support material for a speech. I find bias to be really annoying when I am biased against the topic of the speech, like political standpoints and opinions, so giving a speech with biased support materials will cause some of your audience to tune out. Biased material usually isn't 100% true either, as it can bend the truth to match their ideals. When giving a speech, if you are truly correct in your opinion, you shouldn't have to use material that only supports your reasoning.
I see where you are coming from but what about the fact that being biased is inevitable and you have to be biased in order to have your opinion come across.
I agree when you said bias can get annoying and you would rather stick to the facts. I would hate listening to someone speak and them not even touching the other side of the argument and them basically just saying what they believe. Facts are crucial to a speech.
I agree that biased material can be hard to believe because it can bend ideas. I also believe that it can be hard to listen to a speaker if they are biased, and they only mention their opinion the whole time.
I agree that people should try to not use bias in their supporting material. It is definitely frustrating when I'm listening to someone support their claim with biased evidence because it makes them seem less educated about the topic as a whole. I think that including facts rather than biased support definitely keeps me more focused on the speech and allows me to even consider their view if I didn't agree with them beforehand.