It's rare, but possible to have more than one remote URL in SRC_URI, in which case you won't get a progress bar for do_fetch that accounts for all of them - it'll just go up to 100% for each one that supports progress reporting, which can be confusing if some of them don't - it'll look like it's at 100% when it's actually got a bunch more work to do. Recipes that fetch node.js modules from somewhere other than npm are likely to suffer from this as they tend to have other npm:// URLs in SRC_URI for dependencies. (See also bug 10782.)
It is theoretically possible to look at the number of entries in a SRC_URI and divide that up into sections for each one. That would mean suppressing the individual progress bars for the parent one and that could potentially be quite invasive code. This seems to be a case where the added complexity and maintenance burden isn't worth the minor gain in the interface - > WONTFIX.