I'm asking because all effects I can think of are either (1) to be implemented as SVG filters, which would apply equally to both embedded bitmaps and vector objects - for example, blur and its derivatives (shadows etc.); or (2) should really be internal Inkscape commands, also applicable equally to both bitmaps and vectors - for example, brightness/contrast, color adjustments, etc.
Doing these things via bitmaps may be acceptable as temporary workarounds (one of which I myself use, see doc/inkscape-shadow.README), but in the long run we should really concentrate on doing things the Right Way, which means the Vector Way. I know that, for example, Xara can use Photoshop plugins to process bitmaps, and it's good for eyecandy, but it never appealed to me because of its lack of genericity and overall clumsiness.
I agree, is awful to create effects as bitmaps and force embedding those bitmaps in the SVG file.
I think it's not necessaty to embed the bitmap for generating the effect, we just need to save the parameter used for the creation of the effect into the svg file, to replicate them at rendering time.