With my 70-200mm lens 200mm at f2.8 my DOF is just a touch over 8 feet. At f8 it's 23 feet. If you can't get an entire baseball player in an 8 foot DOF then you have a problem. Having everyone and everything in focus for 23 feet will not make much for much of an action shot.
Firstly, here is an immediate distinction - you are shooting with a zoom lens that maxes at f2.8, the person I made the suggestion to had his max out at 5.6 at 105mm. Secondly, depth of field is variable, its not an equation as if f2.8=8 feet dof. It depends on how close the focus subject is. At f2.8 100mm on my macro, when shooting at 1:1 mag depth of field is about one millimeter, at the infinite focus disdance (~33ft+), dof is, yup, as infinite as the sensor allows at this point. It also varies *quite* a bit if its 70, or if its 200. Your dof will change between your zoom range, I assure you.

If you want dramatic bokeh, keep the sports subject within a much closer working distance, so the dof effect is more dramatic and pronounced. At much closer working distances, f4-8 is more important. Now, if youre just some in-the-bleachers shooter, and you are more than 60 feet from the action already, then yeah - it really makes no difference, go ahead and shoot wide open.

Now, I dont entirely disagree with you though - I merely mentioned f8 because its one stop down on his particular *kit* lens. If he had a pro lens at f2.8, f2.8-f4 shooting is a big deal of difference, even then, on pro lens that max at f2.8, there is still an important function with f4-5.6 shooting, stopping down the lens always improves the optics performance.

I'd also suggest that ISO 100 sports shooting is for the 90's, in this digital age where speed is crucial, take 200/400 a lot more, the insignificantly increased noise (especially on the D90) is definitely a good sacrifice for the extra stop advantage.