The advantage of Handlebars over Mustache, and the reason it's far more popular, is that it does support helpers (and pre-compiling).  This isn't code in the template a la plone's `tal`, it's code that either has to be done in the View (often in a messy way), or code that can be placed in a helper (outside the template) that can be used globally and often in far fewer lines of code.

That's why libraries like https://github.com/elving/swag and http://assemble.io/helpers/ are popular.