@Anonymous

shouldn't you always double-check what gets input into your system?

No, I should control what get's put into MY accounting system.  Allowing an unknown entity to make entries in my records, entries that can cause me problems with the IRS, is foolish.

It is my responsibility to insure that I have documentation to protect me, showing that I complied with the legal requirements.  And keep in mind, most users are not aware of the requirement or the penalty for not insuring the tax id of a contractor is valid and/or whether the contractor is subject to tax withholding.  While the w-9 does not actually check the validty of the tax id, it does put the onus on the person signing the form, and the fact that I have that sworn statement proves my due diligence.

It would be nice if 100% of the working world had the integrity you infer, but even the white collar highly educated crowd doesn't.  Deal with tradesmen, and you will find many that are illegal, behind in taxes, behind in child support and have a judgement against them, or any number of other issues.

The world is connected, and while the US is behnd most countries in this, it is catching up.  The integration of federal and state agencies is happening.  Today it came out that if you owe the IRS more than 51K (with out a payment plan, etc in the works), the IRS will notify the state department, who will

1. refuse a passport application, or
2. refuse to renew a passport, or
3.  confiscate the passport

In TX, if you are behind in child support payments, the court notifies the DMV, and you can not renew the vehicle registration.