Thanks for your feedback. The themes work as expected when released, based on our tests, but there's always the chance of something not right when they are installed, due to some oversight or due to site configuration differences, etc.
About your points:
> when i changed the theme to andreas09 the wiki lost all style information....
This normally doesn't happen; many sites use this theme successfully. I don't know what would cause it at your site. This should be pursued if it's a problem, but I can't reproduce the problem at my sites.
> tikiwiki will fall back to tikineat....
That's right. Tikineat is the default theme in TikiWiki 2; it is more mature as it has been part of the core files for a long time, whereas some of the other 'nicer' themes are new and so are less tested. This will change in TikiWiki 3.
> litejazz . . . forgets if you chose left/right menus to be hidden
> The footer has no background image. Grey text on grey background is annoying.
Oops, the docs are missing. As you can see at http://zukakakina.com/LiteJazz, this theme has styling for custom footer code. But it seems the docs on this got lost at some point. I'll take care of that.
> i compared some files in eatlon and andreas . . . there are a whole lot more differences...
> why are there still unnecessary(?) themes in svn while really good themes are left outside?
To some extent old themes were carried along because nobody objected and because some established sites still use them and wanted the continuity. In the next TikiWiki release though, coming in March or so, the number of included themes will be reduced quite a bit. The second part of the answer is that many of the "really good" themes have a license, such as GPL or Creative Commons Attributive, that is not compatible with TikiWiki's (LGPL) and so the files cannot be distributed as part of the TikiWiki package.
> And a 2nd question is, if i update i.e. litejazz to 2.2svn who could then tell what a user with tikiwiki 2.1.3 is supposed to do?
As far as the Tiki releases are concerned, I don't think the differences in the incremental updates, which are limited to security fixes and major functional bug fixes, should affect themes. For themes that aren't in the Tiki package, I think basically having compatibility with the major Tiki release versions (updating for Tiki 2, for Tiki 3, etc.) is probably sufficient.