I think you would be shocked by the number of people getting EITC...especially in Central Iowa where there are a LOT of people who don't get paid what I would consider a living wage. Why do CPAs do their returns? Because in a lot of smaller towns that's the option available.
I also think you grossly overestimate the ability of the average person to figure out their taxable income. Or to know they may get $2,000 per child. Or to know what qualifies for American Opportunity Credit. How much student loan interest is deductible and when does it phase out? What numbers from my 1098 need to go on Schedule A?
Again...I used to be a tax preparer....in central Iowa (Des Moines) , so no, I wouldn't be shocked. I saw if firsthand.
That said, my experience tends to confirm my suspicion that my clients getting the EIC, were not likely CF forum folks.....which is the audience here (obviously) . Not the general public. So using that as your starting point to refute my point that most people (the audience reading this thread) can do a fairly decent job adjusting their w/h prospectively still stands IMO.