Saving SVGs from GIMP is like saving PDFs from Photoshop.
Sure, it outputs a SVG file, but the editor is focused on editing bitmap images. Most people will get a PNG or JPG embedded in an SVG when saving an SVG from GIMP.
In the past (Its been a while since I've used GIMP so this could be completely different now), saving an SVG from GIMP would first render most everything too a raster image format, then just embed a single or multiple raster images in the SVG, turning the SVG into basically a wrapper around the layers of rasterized images.
Inkscape is intended to work on shapes and not rasterized images. Text doesn't get rasterized before saving, it gets written to the file as texts using a specific font or as curves. A rectangle is stored as a rectangle object with which a border style, fill style, and maybe a filter. Circles, and other polygons are the same.
Later when you want to resize an object stored as a shape rather than a rasterized image, you just scale the shape, there is 0 quality loss. Resize a rasterized image in GIMP to something larger and you'll start seeing artifacts rather quickly. Changing the border color on a rectangle in GIMP would require you to select the area around the rectangle with manually, with a magic wand tool, or maybe a script, then change the color of the individual pixels, overlaying the existing pixels. With antialiasing turned on this can quickly turn into a mess as it blends in with the existing colors or the background. Changing the border color in Inkscape will result in a final image without the mixing of colors associated with rasterized images as the file is really a set of instructions for drawing shapes. Instead of changing the individual pixels directly, you change the command that creates those pixels in the first place.
Inkscape is to GIMP what Flash is to Photoshop or GIMP.
Theres a lot of other differences and a lot of commonality between the two from an outside perspective, but you'll find that if you're editing a photo, you want to do it in GIMP. If you're drawing shapes, flowcharts, and the like, you'll want to do it with an SVG.
I read somewhere, although I can't verify it, that Southpark (The TV show, if you live under a rock) is done using SVG. Even if it isn't, Southpark would be something SVG is perfectly suited to doing, where as doing it in GIMP would surely suck ass for the guys doing the drawing and animation. It'd be relatively simple to do with SVG.