Actually, I still use Gulp to call Rollup.js as it is more flexible and allows me to use features which are not the scope of Rollup.js (e.g. automatic browser reload). My point is, that the exact thing Rollup.js focuses on is very important for the long-term development of JS: Modularization. Yeah, I know thats an upcoming part of the JS standard. But with Rollup.js you can use it today, as it has various adapters for legacy formats and therefore you have a tool which allows you a smooth transition into the future ;-)