Yeah, Corona688 is right. Think about it guys:
Zoom with SVGs:
Load SVG -> Parse SVG -> Render SVG with mipmaps for OpenGL because it only supports bitmaps of some kind and you'd be stupid not to use mipmaps -> check zoom level and pick right mipmap -> render as usual
Zoom with various predefined bitmaps:
Load Bitmaps -> check zoom level and pick right mipmap -> render as usual
Which of these sounds better? In the end, the hardware rendering is what determines precisely what data is actually rendered, so by the time the information gets to the renderer, you are doing the same thing. Hell, if you wanted to implement this so there would be as little lag as possible, the program would load the SVG and pre-render all of the mipmaps and cache them anyway. It is the same thing, only one requires more time to implement.
Furthermore, you will get far more aesthetically pleasing results with separate images defined for each zoom level. As G-Flex pointed out, if I make a ridiculously large tileset in SVG that details fingers, 1: no one will care as they play and 2: it will just gum-up the works when the set is rendered at 16x16: a muddy mess of brown pixels.
However, there is a problem with all of these problems: DF is, as far as we know, in flux as to its final presentation. Who knows what Toady will do? The game is designed out of the box to have a "classic" feel of ASCII based games of yore, and in that context SVG isn't so bad of an idea. It's like a font with fewer restrictions.
But will that remain the case, and if it does, is that desirable? What if it remains that way, but the community develops a complete graphics revamp that makes it have more of a sprite based (these days, hand-held) feel, assuming Toady gets to full graphics support? At that point, SVG support is barely a neutral position, as pixel-based graphics will unquestionably provide a better look, feel, legibility, and modding possibilities (cut to super-simple ASCII style on far zoom-outs, but sprite-like detail on zoom in; with definable zoom-level maps this would be a given).
That means that overall, SVG would be, I am sorry to say, a bit of a waste of time as far as the game is concerned. As a coding exercise, a useful tromp through vector graphics and OpenGL, and a megaproject worthy of Armok, but like so many dwarvenly exploits, at the end of the day your really just killing elves goblins in creative ways.