What are some common use cases for implementing CFML Custom Tag (not CFX tag)? In 3 yrs of my CF exp I've never written one. Would someone please enlighten me, under which use case / situation would one choose custom tag over cfc / udf?
Remember that custom tags were, at one time, the only method available to extend CFML (up until version 4) - UDFs came later (CF 5) and CFCs later still (CF MX). They're not as commonly used as they once were for the simple reason that there are more options.
Custom tags are basically procedural in nature in a language that, with CFCs, become more and more OO in practice. This is another reason that they're not very common.
But there's still cases where they come in handy (but are never required) - mostly for interface work. The ability to create both a start and end state can definately come in handy. A simple example could be a "wrapper" for page content the opening tag might add the HTML header and page navigation while the closing tag would add the footer and end the page.
In this way your page content could be nothing more than:
<cfmodule... >
Page Content!
</cfmodule>
Of course there are other ways to do this as well - but sometimes the classics still have value. ;^)