I spend a lot of time thinking about purity and powerful type systems. Tony and I were recently chatting about how his experience with Haskell, for example, has made him a better JavaScript programmer.
The neat thing about learning those systems is that they have a lot of transferrable wisdom and ideas. Sure, sometimes those ideas don't transfer directly as boundaries within the code, but they can exist within your head as you architect your software.
So, while we haven't really used Elm and PureScript directly (yet!), I'm very grateful for having been exposed to them and their associated ideas.