When building multilingual sites we usually have one of these two user stories:
Dedicated translation team
When the site or the team is big enough or there are many languages, we usually have some people to build and administer the site and some other people to translate the stuff. For this case you'd like to have:
- A 'translators' role without too many permissions as translators may not be technical people or they may be external people hired just to to the translation.
- All the translations grouped somewhere in a few pages so translators can just translate and don't have to browse all around the site to search the strings
- Language oriented pages on which the translator can find all the translatable stuff for a given language
Administrators/translators
Usually for smaller sites managed by a small team and without too man y languages. In this case the site administrators usually do the translations themselves. The best example is a simple bilingual blog on which just one person writes and translates everything. For this case we possibly would like to have:
- On-page translation for most strings or easy links to translate everything. It should be easy to edit any string along with its translations.
- Relaxed permissions because if we have the translation UI on every admin page, this means translators will need admin permissions to edit that pages
- Also we don't mind to have more crowded admin pages in exchange for saving some clicks every time we edit an string. We better have one more textarea for every language in a page than needing to go to the separate translation page for every language and every string
These two different user stories usually take a completely different approach when building a translation UI and many times are not really compatible. I.e. it is nice to have a text field for every language when the site admin wants to create a taxonomy term in two languages, but it's not so nice when you have 12 languages or you don't want the translators to have access to the taxonomy administration pages.
The Internationalization module wants to support all these user stories but on the other side we don't want to have pages that just won't scale for too many languages or a dedicated translation team. So the design principle for this module is to provide the basic API, storage and features without going one way or the other.
This is why we have now this module Internationalization UI to group together all these UI enhancements that will support the second user story, mostly on-page translation for everything. Not too much in there at the moment but contributions are welcomed here.

Comments
partially disagree
I agree what you describe above for issues like [#582438]
But I have to disagree for other kind of issues like [#578360]
I'm not willing to provide UI improvements for neither of the use cases above, since neither of those would be my use case.
Doesn't matter whether the is a "translation team" or not.
Administrators can access administrative pages of translation, taxonomy, everything.
Administrators supervise the team work, can review configurations, revisions, moderation, even when he/she is not a moderator/translator/etc.
1- When a website has a typo or misspelling (in any language) the "webmaster" is the usual person to contact about it.
2- Translators may or may not have skills enough to audit themselves.
3- A website might not be equipped with the very best suit of modules to make translators live easier.
4- Technical translations may not be accomplished by translators.
5- Also, (as stated above) small websites have a single admin which would be also the translator
-- I think there might be found other reasons
PS: for the particular issue mentioned above I have more arguments http://drupal.org/node/578360#comment-2221110