When I use my 11-16 Tokina wide-angle lens at F2.8 on a sunny day, I am often limited by the 1/4000sec exposure limit. It will often force me to close the aperture a stop, to compensate.
The same is the case with my F1.8 500 lens as well.
As for the D300/D90 comparison, I agree with you. The inherent/technical image quality between the two..well there really isn't any.
The D300's 51 AF points and it's superb 3D tracking makes very good for shooting moving objects and keep them in focus at the same time - which is coupled with it's fast continuous shooting. But if that kinda shooting isn't something you do all the time, then it's probably not that big of a deal.
The D90 on other other hand, offers video-mode which can be quite handy for some situations.
For hauling around all day, the D300 is somewhat heavier, but also more sturdy and solid and can "take a beating".
All in all - unless there is a specific feature of the D300 you "must have", there really is not compelling reason to spend 5-600$ more on the D300.