Replacing Identical/X templates
The 'suggestions from translation memory' are sometimes enough. But I do check the Identical translations quite often, especially when an English word can be translated with two or more different words (or grammatical variations of a word) in different contexts. In these cases I find it useful to have an overview of all variations on past translations, not just two. Even if there are not more than two translations listed in the Identical templates listing, the layout shows everything together, which is quick to understand. At the same time I do sometimes adjust some of the other translations for consistency. It is occasionally (not often) useful to be able to look at other similar messages which haven't yet been translated, to gain a sense of how this message is used in different places.
I used to use identical messages to translate similar messages at once. However, nowadays I try to do this for core messages only, because I don't want to start translating extensions and other projects when it could be many months before I have a chance to complete them (as recommended by translatewiki.net).
I would also find it very useful to have a quick way to list or read all messages using a particular word. At present I load all translated core messages and all translated extension messages in separate tabs (and sometimes the untranslated ones too) and then use my browser's 'find' tool to read all the possible contexts of a word, and all their translations.