When is it beneficial to file "Married, filing separately"?

In most cases it is not beneficial for most married taxpayers. In some rare instances where the second spouse has low income and a lot of deductions it may be beneficial. The only way to know is to run tax computations both ways. That is the only sure-fire way to know which is more advantageous. If you do not know how to do this you may want to have a good tax accountant, CPA assist you. Hope this helps.

If one of the people has very high medical expense, or unreimbursed emlployee expenses or some other expense subject to a 2% limit, it might be beneficial to file separately - usually not even then.

If one of the people does not want to be responsible for the other's taxes, filing separately avoids that responsibility in most states.

Other than these situations, filing a joint return is better, sometimes by only a few dollars, someimes by quite a bit.

Regarding the statements that it's beneficial when one spouse has a lot of deductions (Itemized, that is).

When one spouse files Itemized, the other has to as well.

So, unless the spouse without the unusual deductions is able to Itemize, it's not a beneficial scenario.

One of the primary reason to file MFS is to reduce your tax liability.

By filing Joint, you have to combine Income. This could "bump" you into a higher bracket. And if you don't have kids and use the standard deduction, you might be better off filing separate.

The other is to separate your liability from your spouse's.

I know of a married couple that has filed MFS for over a decade even though they have kids. He does not keep accurate books and records and has not filed a return in years. The last time I saw his "records" it was a jumbled pile of crap missing loads of documents. Plus, it's a cash business - vending machines.

So, with a cash business where the spouse didn't keep good records, the wife was willing to forgo any credits because she did not want to be accountable for his thievery.

When a couple is split up and don't want to be responsible for a joint tax debt or in a non-community property state where one spouse has a lot of medical expenses that would be diluted in a joint return.

If you are married to a foreigner, who doesn't live or work in the U.S., and makes a lot of money in another country.

Rarely

When spouses don't agree, don't talk, wont cooperate, won't sign joint return

Most credits are NOT available to those filing MFS