no, actually it does matter. Browser caching is not reliable and only covers around 20% to 25% of requests (according to GoogleMaps JS caching behavior stats). So it matters quite a bit.
It's not just about the requests. Some developers forget that not everyone has 8gb of RAM and a quad-core CPU. Those scripts add up and clog both memory and CPU time.
40-60% of Yahoo!’s users have an empty cache experience and ~20% of all page views are done with an empty cache. To my knowledge, there’s no other research that shows this kind of information...