Javascript is a good programming language, in that it's expressive, fast (much faster than Python), and more importantly it runs everywhere. No ecosystem can grow without developers, and the success of Node.JS clearly shows that developers like a system they can dive into without much prior effort, using skills they already have.
Tessel allows web developers to use skills they already have to do new things. How could that possibly be a bad thing? Just because Javascript has '==' and '===' and both 'null' and 'undefined' and {} + [] !== [] + {}? I swear, everybody seems to have seen the 'Wat' talk and thinks they're now experts on Javascript development.
Javascript, like many languages (even PHP!) can be written in a 'good' way and a 'bad' way. Thankfully, it's not very hard to write in a good way and it lends itself very well to I/O bound applications like webservers. Embedded devices, depending on the application, could also be very I/O bound, waiting on sensors, cameras, wifi, etc.
Javascript is actually one of the fastest interpreted languages in existence and thousands of new programs are being written in it daily. It's easy enough for a total beginner to use yet powerful enough to port Unreal Engine 3 to it. Get over your biases and recognize that JS brings with it something that Haskell, Scala, etc., will never be able to match: large numbers of ready developers.