Now certainly, most people are capable of working on a few things at a time, and indeed, many people will get bored, apathetic and complacent if they aren't being challenged, but the opposite extreme is when a person gets a pile of worked dumped on them, and then has no priorities set for them with regards to completion, they will end up being less productive than the person who only has one thing to do at a time.
My daughter was listening to her MP3 player, texting and talking on her cell phone while sitting in the Easy Chair. I said “Evelyn, dinner’s ready” three times. No response. So I switch the channel to get the score of the Boston College football game. She says “hey, I was watching that.”