Point taken.

- and the lies he's admitted telling - :D

This guy also admitted stealing from a number of people including Manafort.

The big question - What is the value of such a witness?

The same as each and every cooperating witness - provide eyewitness corroboration of other hard evidence.

As finn has posted, he is a corroborating witness, not a 'he said'.
The prosecution has Manafort's bank and tax records, he just needs to connect the dots. Even so, this case is not a slam dunk by any means since there is a chance that there is a juror or two who is a member of Club Dotard.

The Ultimate Betrayal of Paul Manafort
When Rick Gates turned against his former boss, he described a parallel ethical universe where lying and cheating were as comfortable as a well-tailored ostrich suit.

https://www.theatlantic.com/politics...trayal/566945/

Small wonder Donny is freaking out. His underling Cohen could be flipping on him in the same way Gates flipped on Manafort. Donny lives the exact same amoral life as Manafort, where money, power and greed are tantamount.

Let us hope there are no Whell-like idiot lemmings on the jury.

Apart from 'member of Club Dotard' there are also many people who are just averse to this type of witness.

“This type of witness”? You mean someone who is actually willing to tell the truth about his grifter boss? Would you include whistle blowers in your condemnation?

Organized crime cases (which, by definition, includes virtually anything touching Russia or Ukraine;)) frequently use cooperating witnesses. Organized crime criminals tend to hang out with other criminals. It has always been thus.

Not a condemnation. Nothing against whistle blowers. Just ventilating peculiarities in trying to achieve resolution in the justice system.

Many a times jurors just disregard such a witness...a negative character singing loudly because he is promised a deal.

Then, there is jury nullification.