what about them?? they are just one more icon!
Humm not really, in fact the different kind of icons have different kinds of usage, and almost all icon themes treat them has "one more icon".
In fact this is a common mistake that i did myself several times.
For starters action icons are normally rendered with a very different background than most icons, wen they are made, normally in a vector program the background is white in action icons normally it is .... well your favorite window color, that poses one problem the preemption of the limits of the icon can be misinterpreted by the user, thus the usage of solid outline helps in this aspect.
In second place they are small :(, yea has an kind of artist it the worst part, you do a very detailed and perfect drawing, men it looks sweet, but then you render it to an 16x16 pixel map, and that it, 16x16 little squares of solid color, and we did al that funny details that were so important went?
Lost! So if you are doing action icons it is stupid to go in to big details, this is being said by a stupid guy :).
There is an other procedure to the problem doing completely different icons for each size, i really don't like it, in the end it becomes confusing that a menu action icon is not the same has the one in the bar, being they the same.
Third action icons refer to action, it is a "do something kind icon" so you have to do something that implies some kind of movement.