I believe it is only worth doing it if you are really interested in a specific area, e.g. intellectual property, and wish to expand your own knowledge of that area. Perhaps, in work terms, it can be helpful as well, if you're looking to practise in this particular area and you apply to firms that specialise in it. It will probably show that you have a genuine interest and commitment.
Also, I think it may "open up" some other sectors as well if at any point of your degree you doubt whether you want to become a solicitor/barrister, or if you are unsuccessful in finding a job in the legal sector.