Yes, it's a pretty safe assumption.
1. It brings your content closer to the top of the html page. This is great for the spiders. Sometimes when there is lots of js on the page, they never get to your content at all, and you end up with only the js in their database!
3. The js file is cached by the client, meaning that after one download, all other pages will load faster.
>> "using a js redirect in an external file none of the engines will know this if it is external..."
I've heard this. Won't the search engines index the page that you are re-directing to instead of the actual page? <<
No, the spiders almost never call the js file and will not see the redirect URL at all. Right now search engines would have a lot of security issues trying to automatically run your js on their machines to find out what it does.
Google has said they "reserve the right to check your js in the future", but even this technical monster doesn't routinely check js files right now.