Have you seen code that uses .valueOf() for anything good or useful? new String()?
For ES4 (after AS3), we tried eliminating boxing. This is overtly incompatible, a web-breaking change. It won't fly.
Java has primitives that can be autoboxed or explicitly boxed too, which is why JS has them. I was future-proofing for "LiveConnect", which shipped in Netscape 3 (Nick Thompson did the integration).
But I was also in a terrible ten day hurry, so found unboxed primitives easier to deal with in the Mocha first implementation.
If I did not have the "make it look like Java" and ten-day marching orders, I like to think I would have resisted the lazy-programmer second reason. But it's all water way under the bridge now.
Implicit coercions, e.g., '' => 0, were influence by Perl 4. Nuff said!