[spaetzleesser]

“If the only advantage is the 'pre-filling' part, then I much prefer assembling the data (W2, 1099s, etc) myself and file taxes. Period.”

Why? The government gives you all they know and you can then accept, amend or totally rewrite the return. I don’t see the downside to this.

“2) If US IRS sends me a prefilled tax form with erroneous income, say off by 10%, sure I can correct it and file it but now the onus of proof is on the citizen to disprove the error. You might say 'Ok, it is just prefiling it, you can always correct it' - future laws will ask for proof.”

The burden of proof is on you already when you write your return from scratch. It gets compared to what they know already which is the data a prefilled return would contain.