Regular expression has a big use in the url manager when you need to build complex urls, an example when creating SEO urls. Your url might be mysite.com/my-pretty-title-234
you may only need the 234 as that is the id (imagine this is a crud controller). Regex is a valuable tool here for parsing such things.
If sensorario’s proposition is to be implemented, it could be in the form of a default regex (something like this: “.+?(?=/|$)”) for rules without one.