What are the advantages of running Angstrom on Pocket PC over Windows Mobile(some sort of program which has no alternative on Windows Mobile etc)? I mean is there anything particular what Angstrom is capable of and Windows Mobile is not?

Sure - it is capable of respecting your rights and does not require you to agree to draconian end-user license terms.

However, is there any advantages besides licensing?

You get to use the pda as an actual computer instead of as a glorified scratchpad. You get access to the wealth of free and open-source software. Was there something in particular you were interested in?

That licensing implies pretty big advantages - you get to have a media player without DRM... yea, even on movies and e-books. You get viewers which support open formats - which are standards compliant. You get updates which actually track.

The main trouble with discussing software freedom is that so many people have never known freedom at all.
Imagine I have grown up in a prison - it's a nice prison, carpets, windows, fresh air ... but still a prison. I would ask you, "what is the advantage of being free?"

After all, my meals are delivered through a slot in the door for free, the rooms are warm and I have lots to occupy myself with. So what could freedom possibly offer me?