Good writing begins with syntax, and nothing weakens a sentence more than the comma splice. Look at an example:
This device, far from interfering with the law of the Pendulum, in fact permitted its manifestation, in a vacuum any object hanging from a weightless and unstretchable wire free of resistance and friction will oscillate for eternity.
Here the comma splice occurs between “manifestation” and “in”; the result is a fused or run-on sentence where two independent clauses have been joined without an appropriate conjunction or punctuation mark. There are three ways of correcting this: replace the comma with a full stop and start a new sentence; replace the comma with a semicolon; follow the comma with an appropriate conjunction. The writer of that sentence, Umberto Eco, chose the third option: his comma is followed by the conjunction “for.”
Pick up any newspaper or magazine, and you will find comma splices galore. They occur even more frequently in the compositions of school pupils. Look at this example: “Suddenly there was a knocking on the door, I could feel my heart thumping in my chest, the knocking got louder and louder, I went downstairs clad only in my flimsy pyjamas.” I drew the pupil’s attention to the three comma splices, and he corrected them thus: “Suddenly there was a knocking on the door. I could feel my heart thumping in my chest, as the knocking got louder and louder. I went downstairs clad only in my flimsy pyjamas.”
I suspect that the main reason for the proliferation of comma splices in popular writing is the near demise of the semicolon as a punctuation mark.