Focusing on images exclusively, here is the difference between two page loads, one with image errors, one with WP-RUBI replacement, as recorded on Google Chrome Developer Tools console (no throttling).
With images replaced by WP-RUBI:
In short, as illustrated by the extended green bars in the first image, Chrome attempts for around three quarters of a second (ca. 750 milliseconds) to load each of two broken images, though it saves itself and the user some time by running the two processes (among others) concurrently. Loading the replacement images, by contrast, requires a single operation of up to 10 milliseconds (no green bars). The effective difference in page load is around of 0.4 – 0.6 seconds, depending on whether you’re measuring in relation to “Finish” or “Load.” The time difference stays in the same general vicinity or proportion across emulated 3G, 4G, and WiFi environments.
This difference on its own terms would be even more important in relation to WP-RUBI if our main emphasis was delivering faster page loads (better experience) for normal human users, and the differences might be even more pronounced on different browsers. The difference is also said to be important in relation to Search Engine Optimization (SEO).
Either way, in contemporary web terms, half a second of extra waiting is an atrocity: Developers have been known to spend hours shaving milliseconds off of program execution as well as image rendering in the eternal quest to produce fast loads both for user convenience and for SEO.
As I have noted elsewhere in WP-RUBI documentation, errors are also said to be independently significant to rankings, and another set of errors will also accumulate in relation to “broken anchor links” of the sort that in many WordPress installations will accompany the broken images. When the Search Engine “crawls” your old posts, it travels from link to link, and, if the link fails, an error is reported, producing the additional problem for developers of site error logs sometimes getting stuffed to the point of uselessness with them.
This benefit or set of benefits from using WP-RUBI is additional to avoiding “the $8,000 mistake” of using unlicensed images, but for some sites will be much more important.