I still think that MPs should be able to keep a toe in the water of their profession. A political career is uncertain and many MPs need to find another way of earning their living within 5 or 10 years of entering parliament and whether they are a QC, doctor, teacher, or plumber, if they haven't kept up todate in their profession during that time, once out of parliament they are unemployable.
What should be limited is the number of hours a session, or week they can devote to outside interests. Most people work a 40 hour week and why should MPs be expected to work longer hours? This means they could reasonably devote 10 hours a week to other pursuits, whether, legal work, plumbing, playing golf or making matchstick models, without it interfering with their Parliamentary work and service to their constituency.