Phil are you bridging out using the CLR bridge for an http client? If you don't mind sharing about your implementation I'm curious.
As far as utility goes, Underscore is the most useful library to me. It's almost canonical in the web development world and the subject of more than one book. I would say it's not going away. I completely disagree with the idea of using the built in objects over JavaScript ones. JavaScript is what's not going away. The code will work in any JavaScript engine whereas the TestComplete objects only work in TestComplete. You can always find code on Stack Overflow or the like to do something in JavaScript.