Omitting a Conjunction in a Series

After doing the sentence error exercise, I've learned a new rule that I thought I would say a little more about. This is number seven on the sample questions page:

They said they plan to sleep in their SUV, a five-year-old Chevy Tahoe, into which they’ve packed boxes of important documents, bags of clothes, some food

I mistakenly added the word "and" between the serial comma and the last item in the series, which is actually unnecessary. Below is an excerpt from the ClearWriter website.

"Dropping a conjunction has the opposite effect of adding a conjunction: creating a series that is not exhaustive, but a mere sampling of possibilities. It also makes your reader see the parts of the series as more separate than joined.

This 20th century is baffling, difficult, paradoxical, revolutionary."

:)

"For" as a Coordinating Conjunction

What are the stylistic conventions of using "for" as a coordinating conjunction:

I left early, for I felt sick.

They hired him, for his application was superior to the other applicants.


I ask this because I used to employ this technique a lot when I was younger. I stopped for three reasons: it did not see like a commonly used; it seemed like a cheap transition; it stilted the prose in some cases. I am wondering what copy editors or editors think about this construction?

But I was told to never do that!

"Never start a sentence with 'but'!" Is this true? I've seen many instances in writings where sentences are started with the word "but." And what about "and," "a," and "an"?