I think it really comes down to what you are testing, I work primarily in the Oracle OSB/SOA space, lots of database, SOAP and REST Web Services and FTP operations, all mostly back end stuff and I can get by with an old version of JScript, I'm just glad work didn't decide to use VB Script!
Acutally, the most important thing in Test Complete for me is the abliity to write class libraries in C# and access them via the CLR Bridge, this has proven quite helpful to me.
But yeah, in your case I can see how using JavaScript libraries and having a modern JScript engine would be a huge advantage.
Phil