To me, sending a thank you email does two things: reiterates interest and keeps you in their short-term memory (consider that the company may have interviewed several candidates in a few days).
I've sent thank-you emails in the past. Never longer than two sentences, but it was always of the flavor, "I enjoyed speaking with you today, I hope to discuss the position more." I can't really say how much it helped, but I know it didn't hurt as I've had a couple job offers that way.
