my own feeling is that in a 'general' coursebook (e.g. not a book made for a specific skill), there should be a good balance of all the skills, and that this does ot necessarily depend on the topic of a unit.
However, there are two caveats to this: in the first place the sequence of the skills and/or the way they are interlocked may have alot to do with the topic (or at least with the original stimulus to get a topic going). In other words, if you start a teaching sequence with an audio interview, that will determine, to some extent, how the skills are sequenced.
Secondly, I am a big fan of writing, but in some situations neither teachers nor students want too much writing. On the other hand, where coursebooks do take writing seriously many teachers and students are very grateful!
