From JSCript's Wikipedia:
JScript .NET can be used for ASP.NET pages and for complete .NET applications, but the lack of support for this language in Microsoft Visual Studio place it more as an upgrade path for classic ASP using classic JScript than as a new first-class language.
If you were after first-class scripting capability, doesn't PowerShell fit the bill?
The thing is, JScript.Net creates an executable file. So you could write a full-fledged program in it and deploy it rather than just a script with PowerShell. Granted, I don't know of anybody who would want to do that, but apparently Microsoft believes there is at least a handful that do.