Few grammatical usages from nytimes - please discuss

[1947]

1.usage of and followed by ;

In the wake of a devastating financial crisis, President Obama has enacted some modest and obviously needed regulation; he has proposed closing a few outrageous tax loopholes; and he has suggested that Mitt Romney's history of buying and selling companies, often firing workers and gutting their pensions along the way, doesn't make him the right man to run America's economy.

2. usage of two dashes - xxxxxxxx -
Wall Street has responded - predictably, I suppose - by whining and throwing temper tantrums. And it has, in a way, been funny to see how childish and thin-skinned the Masters of the Universe turn out to be.

3. usage of single dash
Actually, before I get to that, let me take a moment to debunk a fairy tale that we've been hearing a lot from Wall Street and its reliable defenders - a tale in which the incredible damage runaway finance inflicted on the U.S. economy gets flushed down the memory hole, and financiers instead become the heroes who saved America.

[Kasia@EconomistGMAT]

As to the usage of a single dash, they are very often used in informal writing. A dash can introduce an afterthought, or something unexpected and surprising.

e.g. We'll be arriving on Monday morning - at least, I think so.

Semi-colons, on the other hand, are sometimes used instead of full stops, in cases where sentences are grammatically independent but the meaning is closely connected.

e.g. Some people work best in the mornings; others do better in the evenings.

Semi-colons are also sometimes used in lists to separate grammatically complex items.

[1947]

Thanks for responding Kasia.
1.When you say single dash is used in informal writing...would it be correct to say that its usage will not considered fine in GMAT.
2. Can you please comment of usage of 2 dashes. My understanding is that all the text between the dashes is a modifier.
3. One doubt on semi colons.
Some people work best in the mornings; while others do better in the evenings.
Would this usage be gramatically fine or can we replace ; with comma here.

Thanks again.

As to the usage of a single dash, they are very often used in informal writing. A dash can introduce an afterthought, or something unexpected and surprising.

e.g. We'll be arriving on Monday morning - at least, I think so.

Semi-colons, on the other hand, are sometimes used instead of full stops, in cases where sentences are grammatically independent but the meaning is closely connected.

e.g. Some people work best in the mornings; others do better in the evenings.

Semi-colons are also sometimes used in lists to separate grammatically complex items.