ya, if you refresh the background, it is going to refresh everything.
Think of a map as a series of transparent layers one ontop of the others. If you want to refresh to top layer (foreground), no problem, just remove the top layer, throw it away and make a new one and place it on top. If you want to refresh a middle layer (geo selection for example), you throw away that layer and everything on top, graphics, graphics selection, foreground and redraw all those, you keep those bellow. The background is like the bottom layer so there is not much to save with a partial refresh, specially since you don't pass in an extent to limit it (null means refresh everything.) You might want to look at basemap layers to improve rendering.