[klibertp]

A sane, powerful type system is actually enough of a rationale and a selling point, at least for me.

Also, allowing side-effects while still providing the mentioned type system makes it potentially much easier to integrate with existing JS code.

[cies]

Another reason for it being "much easier to integrate with existing JS code" is strict evaluation.

Haskell is lazy, which means using GHCJS to compile it to JS produces code full of explicit thunks (think of promises). Both Elm and PureScript are compile-to-JS-languages that inherit a lot from Haskell, but not it's laziness.