Filing jointly and separately really depends on your situations.
In the case above where one spouse has an income and the other doesn't it makes sense to file jointly. Why...because there is a bigger tab bracket if filing jointly.
0-$14,600: Pays 10% over $0
$14,600 - $59,400: Pays Flat $1,460 plus 15% of the amount over $14,600
$59,400 - $119,950: Pays Flat $8,180 plus 25% over 59,400
The ranges are wider that filing separately so if one spouse only works, its almost a discount.
Now, if you both work, what will happen is that one (primary income maker) will be taxed at
0- $7,300: Pays 10% over 0
$7,300 - $29,700: Pays $730 plus 15% over 7,300
$29,700 - $59,975: Pays $4,090 plus 25% over 29,700
$59,975 - $91,400: Pays $11,658 plus 28% over $59,975
And from what I hear, the spouse then gets taxed at the percentage where you stopped. So you make $90,000 and she makes $1. You pay "$11,658 plus 28% 30,025" and he/she pays "$.28". or 28% of $1.
So, you might need to do the math to see which is lower. But most time, just eyeballing it will tell you which will save you money....unless you have to pay Alternative Minimum Tax...then in that case, go see a tax prepaper.
I know this is a little short in explanation, (since it didnt go into deductions) but if anyone needs clarification. I can repost or just pm me.