You can currently import paths and you can create custom svg gradients.
From a photo manipulation perspective a vector layer would be very useful for clipping layers or several layers under the same vector path; along the fact that a vector layer can (or could) modified in any moment and it should also weight lesser in the memory.
It can be also useful for creating graphical elements on top of pictures or raster layers as well.
With SVG support you may also take advantage of SVG fonts & gliphs properties.
It seems natural having SVG layers support on GIMP but maybe there are technical issues or it is simply underestimated, I can't tell you...