On a multilingual website, why would I use the i18n_menu module to translate the menu items of a menu instead of simply making separate menus for each language? I read Gabor Hojtsy's excellent article on the menu system in D7 but I still don't understand in what sort of use case I would want to use i18n_menu vs. simply creating separate menus for each language and setting them to display only for their respective languages.
You have a lot more flexibility when using one menu for each individual language. It's a bit more work to setup but it definitivly pays out in the long run.
These are the points that are blocking me for using one translatable menu:
Bottom line is when you're sure that you have a fixed menu that doesn't need flexibility it's OK to use i18n_menu.
When building a site that could be expanded in the future and has differences between language content avoid the use of i18n_menu and set up a menu for each language. (You can set a fixed language in order to do some ajax magic and only show the menu items available for each language when creating new content.)
I think the main interest of going for multilingual menu vs one per language is the maintenance cost.
It of course depends on the number of languages you have to manage. Two or three is OK, but if you have say eight languages to set up, things can become very complicated with several menus.
Also, node related menus are hard to handle with non i18n solutions.