However, according the HTTP archive websites make about ~90 requests per page [1]. The average browser does about 6 requests per hostname, and 15-20 total [2]. In this situation, our critical path has at least 4 requests, thus we are paying 4xRTT in just latency.
Here's a pretty good post on how latency dominates page load speed after a certain point: http://www.igvita.com/2012/07/19/latency-the-new-web-perform...
By bundling resources, we reduce the number of requests, and the number of round trips.
Disclaimer: I work at Mobify with Peter who authored the post.