If by moderated you mean that each post has to be checked by someone before becoming live on the system - then NO. The power of Mumsnet I feel comes from the fact that you can post a message and quickly have other users views, comments, suggestions. Google also loves that concept - have you seen how quickly Google will index a new message thread, it's quite incredible how you can write a message - then google yourself a few minutes later and see your message in the Google search results.
Why am I going on about Google you may well be wondering... well, I suspect that a lot of people still find out about Mumsnet, due to them doing an internet search - and at present Google is the King of Internet Search. Google brings visitors to the Mumsnet site for free... it does not cost Mumsnet anything (other than letting GoogleBot have free reign of public parts of the webservers - so there is a 'bandwidth' cost... anyway starting to get to techy ). In return Mumsnet allows Google to quickly index new content, which GoogleBot feels is of high value and thus quickly ads it to the Google Search Results. Parents then search the internet for something, and Google provides a link to Mumsnet... the loop keeps going round and around.
If posts were moderated prior to being made live, then I feel that Google may start considering the site to be of less value and thus not index new threads as quickly, which may mean that less parents find Mumsnet for the first time.
I feel that all Mumsnet users act as first-line moderators - we are all able to report a post that we feel is concerning to us.
Following a post being reported, someone then takes a look at it (or maybe several people, I don't know how this part of Mumsnet works).
Maybe there are not enough people in that message loop, for there to always be at least one person, if not a few people who can take a look at reported posts and make a decision as to what to do with that post (such as deleting it). Or maybe when certain posts have been reported in the past, those with the power to delete have not agreed that the post meets the criteria to warrant deletion.
Keeping a post live may well upset someone. Deleting a post may well upset someone.
Is there really any Win Win solution?