I fill probably 10 positions a year. I never ask for a cover letter, but if someone sends one I'll pretty much always look at it. I will say that I don't place much weight on the cover letter content unless it specifically highlights something relevant to the position not sufficiently covered by the resume. Usually I just look at the cover letter as a secondary confirmation that the person can write reasonably well and knows what the position is about.

However, I pretty much never read thank you/follow-up emails after an interview and don't care if candidates send them or not.