According to some posts Ondemand governor should be the best even in battery mode because the cpu can sleep longer if the tasks get finished sooner.

This is probably true but at the moment it isn't because Ondemand even scales up if the cpu doesn't has anything to do.
This might have something to do that the governor can't anticipate how long or how much this task will need so he scales up all the time if the cpu is used but this is bad in battery mode.

So either there is a bug in Ondemand governor which should be fixed or if this happens purely out of performance reasons there should be an additional mode for battery where it only scales up if the cpu can't handle this with the lowest task.

I know that there is the conservative governor but it is too slow, for example it takes ages to scale down.

With powersave the battery time is increased significantly (at least 1W less) and for most tasks the lowest frequency is fine.

I mean in this idle case of the screen shot it doesn't make any sense to scale up all the time but even if you play a video there is no need for the highest frequency and especially not the turbo mode.

I mean why are there more than two frequencies if only they get used?

I am using openSUSE 11.4 with Kernel 2.6.37. I had the same problem with Vanilla 2.6.39.
My processor is a Core i5 of the first generation.

If you need any additional information let me know.