The "rule" of thirds is a little misleading in the title - it really is more of a "guide" of thirds.
Of course, getting the subject/main focal point on one of the guide lines or on one of the red spots in the image above more often than not leads to a more aesthetically pleasing image and therefore it is a handy guide to have.
This, however, is only a guide, as
this famous image by Sam Abell demonstrates. The prow of the boat is centred in the lower half of the shot and the horizon is bang on the middle. Sounds like it should fail the aesthetic test but the reality is that it is a great shot which works very well.
Rules are made to be broken.