The SSL MAC is valuable even in the absence of enemy action.
In "Performance of Checksums and CRCs over Real Data" Stone and Partridge estimated that between 1 in 16 million and 1 in 10 billion TCP segments will have corrupt data and a correct TCP checksum. This estimate is based on their analysis of TCP segments with invalid checksums taken from several very different types of networks. The wide range of the estimate reflects the wide range of traffic patterns and hardware in those networks. One in 10 billion sounds like a lot until you realize that 10 billion maximum length Ethernet frames (1526 bytes including Ethernet Preamble) can be sent in a little over 33.91 hours on a gigabit network (10 * 10^9 * 1526 * 8 / 10^9 / 60 / 60 = 33.91 hours), or about 26 days over a T3.