I think the risk would outweigh the benefit. Sure it's *possible* you delay the gunman, or maybe even get in a super lucky shot, but I suspect that it would more just piss them off and make you more of a target. If you have nothing else and they're killing everyone as they go, sure having something is better than not, but again, if they're looking for certain targets, by fighting back you just draw their attention.

I suspect most people would miss with their cans, or they wouldn't hit hard enough (I think only high schoolers would have any chance whatsoever of having enough strength to use them potentially effectively, and again, it's a crapshoot whether they do or not).