As you write your pages, you can choose to make a page Assembler-driven via pipeline invocation versus making it ATG-driven via servlet bean invocation is based on:
The amount of the page’s content that must be configurable by a merchandiser. Pages that must be heavily configurable by a merchandiser are good candidates for being Assembler-driven.
The number of URLs on the resulting page that should be constructed as Endeca URLs. Pages that contain many URLs that will result in calls to the MDEX should be constructed by the Assembler, so that those URLs are properly formed. For example, the category page includes a facets rail on the left side that consists of links backed by Endeca URLs. These URLs should be constructed by the Assembler API.
