#Translatewiki.net offers localisers an environment where the technical vagaries of updating the code is not their worry. This is a major attraction. Another important tool for localisers is documentation for messages.
When messages are to be translated, it is often not immediately clear what a message is to achieve. The order of variables in a messages may have to change depending on the language. This is where documentation makes a big difference.
For some applications like Status.net developers document the messages and this seriously affects not only the flow of localisations but also their quality. It is an investment well worth it because great localisations make for a great experience.