Isn't picturefill a waste of time?

The point of picturefill is to load a smaller image when the bandwidth of a visitor is lower, or to do the opposite when an image is larger. Since screen size and bandwidth are completely unrelated items, it seems like this solution is extra work, extra markup, and impractical to apply to a site that is already created.

Yes we are assuming that a smaller screen means lower bandwidth, and in most cases that's probably close to correct, but why cant we just use something like this:

<img class="ourimage" src="img/smallestimage.png" alt="Image alt name">

/*larger screens, 600px and above */
@media only screen and (min-width: 600px) {
.ourimage {content:url("img/largerimage.png");}

Then just repeat that scaling up to however large we have. This is still solving the problem in the wrong way by attacking screen size instead of a .js solution to detect bandwidth, but it makes much more sense than using .js and inserting HUGE image tags on the DOM.

I have done this and it does work. I found the content:url css on stackoverflow. http://stackoverflow.com/questions/2182716/how-can-we-specify-src-attribute-of-img-tag-in-css

Any thoughts? I simply WILL NOT use picturefill unless it makes more sense to do so... Or if I want an extra js query, I would rather use one that detects bandwidth for the correct solution.