Yes, you could provide multiple bitmaps in a website now, but in a convoluted way (with some custom javascript checking for resize events etc). In the icons case, it happens automatically.

As for SVG, client rendering time would be insignificant for most case (for a desktop machine at least, they have CPU to spare). But currently used IE version (> 6) for one don't support SVG. And the main problem are bitmap assets such as photographs. Those cannot be done as vectors.