But in Manafort's case, it's certainly a risk.
The prosecution offered not only people who testified that he committed crimes, but extensive documentation of bank fraud and tax fraud. There's another explanation, though, one well put by Franklin Foer, who has reported extensively on Manfort:
"I have spent the last couple of days pondering who Manafort would call in his own defence. I couldn't come up with any good names. And apparently neither could he."
Going on the stand himself was probably never under consideration, since cross-examination would have been a nightmare.
It's unclear whether Manafort had any fact witnesses who could refute the evidence that was offered in the prosecution's case. And there may not be anyone around who would testify to Manafort's sterling character, both because few people want to be associated with him today and because he has long been known as a particularly immoral schemer, almost a walking caricature of the mercenary lobbyist willing to do anything for a buck.
It's unclear whether, even if they had wanted to, his defence could find someone to stand up and say Manafort is a great guy who would never do the things he is accused of.
From the beginning, there has been a question hanging over Manafort's case: Why won't he flip?
After all, other Trump aides have when faced with possible jail time, and Manafort is facing more than anyone.
There's a real possibility he'll never see another day as a free man.
One popular explanation is that he's afraid that if he tells everything he knows, some people in Russia would become displeased enough to kill him. To take just one example, the oligarch Oleg Deripaska, whom Manafort supposedly owes US$19 million, is the kind of guy whose enemies have a short life expectancy.
So Manafort may have decided that it's better to take his chances with a jury than to find a strange substance smeared on his door handle one day.
It's also possible Manafort really has nothing to offer Special Counsel Robert Mueller about Trump, that his activities, criminal though they might have been, never actually involved the President. That would mean he has no one to flip on.
But let's imagine for a moment that he knows something incriminating about the President - or even that the President isn't sure what Manafort knows, but suspects that he might know something.
This, by the way, is Adam Davidson's extremely plausible theory of Russian kompromat: Trump acts the way he does toward Vladimir Putin not because he knows Putin has damaging information on him, but because he just isn't sure what Putin might have.
Would Trump actually go so far as to pardon Manafort, given the firestorm of criticism he'd get?
There are some lines even Trump is unwilling to cross.
For instance, while he complains loudly about Attorney-General Jeff Sessions not being able to protect him by shutting down the Mueller investigation, so far he hasn't actually fired Sessions and replaced him with someone more pliable, presumably at least in part because his aides have convinced him that doing so would be a political disaster.
At the same time, Trump has spent the last 15 months since Mueller was appointed trying to discredit the investigation, in a campaign designed less to persuade the broader public than to convince his base that it is a witch hunt from start to finish and therefore everything it produces, no matter how factual and supported by evidence, should be ignored and discounted.
He has obviously calculated, and rightly so, that if he can keep that base firmly behind him, Republicans in the House will never vote to impeach him, and even if Democrats took control of the chamber and did so, Republicans in the Senate would never vote to convict.
You can already see the argument he'll make: The whole thing is a witch hunt, the charges are bogus, the jury was a bunch of Angry Democrats, and I'm intervening in the interests of justice.
Trump also seems to genuinely believe that the investigation is unfair, and pardoning Manafort would be a great way for him to both assert control and stick it to Mueller.
It's important to remember that no matter what the jury in this case decides, it's only the first of two trials Manafort faces.
The next one, in a federal court in Washington, DC, will deal more directly with Manafort's relationships in the former Soviet Union.
That's when Trump may start feeling the heat and feeling oppressed, and look for a way to let everyone know who's really in charge.
And that's the day Paul Manafort, sitting in his jail cell, is fervently hoping for.
