Is Ember worth learning? Let's see.
It adds no value over anything that we already use or what I see being used elsewhere. It's just another library that doesn't add any significant.
There is not liquid job market with good salaries or day rates with Ember at the moment, you may be able to find one or two roles in London that ask for it but it's certainly not liquid and therefore could not support a contractor.
Embers approach is not radical therefore there's not a lot to learn here. I'm not stuck in a Java rut. You can write modern, slim, fast, concurrent Java. You don't have to be stuck in the days of massive J2EE government projects. The buzz around modern Java in London is massive.
You're right in saying that I could do other things for money but I love software development and I love that people are willing to pay be quite large salaries and day rates to do it. My career isn't money focused but money is a part of it. Why shouldn't I have a big house and holiday often just because I love writing code?
Also, seeing Java devs as being 'stuck in a rut' shows massive ignorance as to what is actually happening in the Java world right now.