I feel you, I agree its not something id do all the time. It really does depend on goals. College football players for example do perform swiss ball DB Bench press about once a month during the off season to work "more" muscle groups. Doing this with 120lbs in each hand helped my overall bench press when I was at the JC level. (bowl champs 07/woot).
But overall I agree that Switching up routine and attempting to hit your core muscle groups from a different angle is a good way to work on weaknesses, which is a good way to gain strength when you hit a Plataea on one of your core lifts.
this article is a great explination of how Sport specific training can benefit from working with stabilizer muscles. The Truth About the Bench Press- http://www.T-Nation.com/free_online_article/most_recent/the_truth_about_the_bench_press_1
Courtesy of T-Nation articles.
Also this sort of training is something that goes well with double days and athletes whoo really do have the time to develop such core stability. It is not something I would do with someone who wasnt already on a great 4-5 day weight training program.