Its not desperate if you don't, but it should be done periodically, every 2 or 3 years as a minimum, really. It gets out all the dead material that's built up, and people who scarify usually aerate and feed their lawns too - lawns which are cared for in this way don't get certain diseases and the appearance is improved. Looks pretty dreadful for a while after you've done it though - best done in autumn.