still i think the speedboost is quite useful if you do something with small particles, where you use thousands of pointlike particles instead of some big ones.. and you don't actually need a texture, but you only need the coords in the pixelshader, and there you can do what you want (rendering a circular "glow"-dot i've done on my gf2.. was 3 times faster as doing with a texture..)
you could:
use r as a 1d texture lookup into a luminance-alpha map storing sine and cosine, or so, or an rgba map storing a rotational matrix, or what ever.. and like that rotate per particle
i think they are useful, but not a simple replacement to billboards.. they are point sprites. nothing more.. sprites on points..
