I'm trying to use namespaces and have got them working but I am a little confused. Am I right in saying the advantages are:

1. Auto load classes
2. Helps keep classes organised
3. Prevents class name clashes

Correct? Here's where I'm confused:

1. Why can't use declare namespace aliases once in a file (e.g. namespaces.php) and then the aliases still apply in any files that include that file? Do I really have to write use or use fully-qualified namespaces on each file? Am I missing something? Is it because namespaces should never change once set?
2. I am using to PSR-0 standard so I have all my namespaces like {Company}\{Package} Is that a good way of doing it? So is the idea if i am Microsoft and I am using namespaces so long as I start the namespaces with Microsoft I know I will never clash with anyone else? (Unless of course they steal my company name )
3. Is the idea if you want to use another library that uses namespaces you can simply copy a folder into your source?