In the past, a taxpayer could use a "more likely than not" opinion from a tax advisor to avoid penalties. A "more likely than not" opinion was supposed to be an opinion from the tax advisor concluding that based on the current law there appeared to be at least a 50% chance that the IRS would agree with the position that the taxpayer is taking in regard to the transaction. If the IRS disagrees with the taxpayer's treatment on audit, then the taxpayer is at least protected from certain penalties.
The use of the "more likely than not" opinion is not intended to give assurances about the tax position itself. It's only a 50% opinion. It is used to protect from penalties. I think one of mbozek's primary points (?). If a taxpayer wanted assurances about the tax position, that is a different issue and they had various alternatives. They could get advice from their own consultants about their position. Or they could go for an individual PLR.
The problem is that there are now lots of "more likely than not" opinions out there that really aren't very good. In some cases, a marketed opinion is being used by a variety of different taxpayers in different factual situations, so it can't possibly be specific enough to comment on any individual's tax position. Some are just discussing an issue at a very high level. Some are only discussing a related issue -- and not the core issue on which the IRS would make its decision on the tax result. And frankly there are some out there that are probably very questionable in regard to whether there is a 50% chance that the IRS would ever agree.
As mbozek notes, the Jobs Act now changes that. It requires certain standards for those opinions in cases where there may be potential for tax avoidance or evasion. Circular 230 provides the details of what those standards are. They don't apply to all opinions. It's primarily those where the "more likely than not" might be questionable -- because the person issuing the opinion is not looking at the specific taxpayer's facts, or because its a listed transaction (one which the IRS has highlighted as being aggressive...). The Circular also allows opinions to be caveated so that they can't be used as "more likely than not opinions" for avoiding penalties.
http://benefitslink.com/taxregs/td9165.pdf