There's no doubt that there is a real danger with allowing under-experienced instructors to teach multi-ratings however as usual it has to be balanced against cost. If all multi-instructors had to have 500 hours multi time then there would be a real shortage of instructors and the cost of gaining a multi-rating would greatly rise. Would you pay an extra £500-£1000 for your multi rating for flying with a more experienced instructor? Some would but I expect that many wouldn't.