I just a while ago committed feMerge support in Inkscape SVN. Now, we have all the parts required to render the example image in SVG specification!
Several other example files are now rendered correctly, too. This filter primitive doesn't bring anything really new, because everything it does, can be done with feComposite, feBlend or even with the simple alpha compositing used in SVG outside filters. It's just an easy way for filter creators to composite several images into one.
Also, check out the Filter effects page in Inkscape wiki. There's a nice listing, which filters work and which don't. There really is a good amount of filters that already work.