Great post.
I was on self assessment for ages (not out of choice) and it was a pain in the arse for me as I didnt have loads of tax offset activities or shares etc. I am now considering this option again as (whilst on it) I got a rebate every year.
If everyone decided to do this it would probably mean that HMRC would grind to a halt
My wife is self employed and I do her self assessment online.Unless you have 'complications' it is pretty straightforward and you get to know how much tax and n.i. you owe immediately.