NOTE: Unless you already know what a comma splice is and why it is usually considered an error, please read my article "What Is a Comma Splice, and How Do I Fix It?" The current article might be confusing if you do not read that one first.
Barbara Wallraff, who writes the delightful "Word Court" column on the back page of The Atlantic Monthly, has recently published a book on the correct use of language. The book's title is the same as her columns, so I'm guessing, as I've not seen the book yet, that it is a compilation of "Word Court" columns.
The current Quality Paperback Book catalogue quotes from her a wonderful line about comma splices--"Take this sentence, for example: 'It's not a comet, it's a meteor.' According to Wallraff, 'punctuating this sentence with a semicolon would be like using a C-clamp to hold a sandwich together.' "
There are times when a comma splice is a justifiable stylistic device, not an error.
ACCEPTABLE COMMA SPLICES
~1. If the independent clauses are very short, especially if the subject is the same for both clauses, then a comma splice is probably acceptable.
I came, I saw, I conquered.
~2. When fairly short independent clauses express contrast, a comma splice is often the most effective way to punctuate the sentence. This is especially true if the first clause makes a negative statement, the second an affirmative one, or if the first clause is affirmative, and the second is negative (as in one form of question).
~This is my father, that is my uncle.
~Some students find writing easy, some find it excruciatingly difficult.
~It's not a comet, it's a meteor.
~We aren't visiting Pennsylvania this year, we're spending the summer in Florida.
~You saw that movie last night, didn't you?
~It looks as though we're in for a tornado, doesn't it?
~You've been to Europe, haven't you?
Ironically, many pedants who declare death to all comma splices do not even recognize the comma splices in those last three sentences, but if you check the definition of a comma splice, they certainly fit.
But--just as in my article "What Is a Comma Splice, and How Do I Fix It?"--I must warn you that it is usually better to restructure a sentence to avoid even a justifiable comma splice, simply because so many teachers, editors, and readers don't understand the issue well enough to realize that you have not made an ignorant error.
Sure, you could strike a blow for reasonable usage, but those people are going to be judging you harshly, and you may not have the opportunity to argue the correctness of your sentence. Besides, in my experience there is nothing harder than trying to argue a pedantic know-it-all out of a rule he believes to be all-encompassing. I save my energy for less frustrating, more productive pursuits. (Of course, since the comma splice cops don't even recognize questions like the last three examples as comma splices, you can safely use such sentences without fear that you will be sneered at.)
Another point in favor of avoiding even acceptable comma splices is that many writers routinely make comma splice errors, so only the most sure-footed should attempt to negotiate such rocky terrain. If you aren't absolutely sure about what is right and what is wrong in comma usage, it's probably not a good idea to attempt an acceptable comma splice, because you might end up with a comma splice error after all.