It seems most of the confusion is due to the various other items across separate marketplaces within Envato. i.e. Photodune, Graphicriver etc. where the same licenses apply.
Even if you put up a website built around a theme/plugin/script element, just because people are visiting your website does not mean that you are making available the theme to end users to download and use. The usage of these items, requires the end user to be provided the package you purchased and then your end users installing this on their servers. Merrily interacting with your website does not constitute usage.
On a website, you visit it for a specific purpose, you pay for a specific service where service is not for end users to see the theme/plugin/script element you purchased.
The way I understand the licensing, reasons for an extended license on Themeforest and Codecanyon are if you are developing a product, that does not conflict with the item purchased, so it needs to be a larger product that does something not related to what you purchased that you are later reselling.
For instance, if you purchased a theme then your end product for sale cannot be a theme but it could be a Customer Relationship Management application, a Content Management System, an Enterprise Resource Planner and so forth.
If you purchased a mega menu script, then your end product shouldn't be another navigational system such as a WordPress plugin of the same or yet another slightly enhanced navigational system. The end product has to be a larger piece of work, like a theme or a web application that you are later selling. In these cases the extended license is useful.
Bottom line, get the extended licenses if you want to integrate the purchased item in a larger product you will be selling. For all other purposes the regular license has you covered.
This is also why the extended license is expensive and sells very little because it covers a very niche use case where the item you purchased adds value to a product you own, wherein you are profiting repeatedly by having payed a one time fixed fee for the smaller work that makes up your product.
IANAL by the way so contact support for a better understanding.