It is not really necessary if you have the right talent and will be satisfied with mediocre salaries. But if you really want to get into a position in life, M. Pharm will definitely take you there lot sooner than the B. Pharm.
3 Reasons to do M. Pharmacy:
1. Higher salaries
2. Personal satisfaction
3. Respected in industry
But also remember.. A 3 year experience with B. Pharm is equal or higher than that of M. Pharm degree. There will be a situation in the industry where you'll be placed under a M. Pharm person. And this might lead to some ego problems (which happened to a lot of persons i know). Many people who opt for a job after B. Pharm will return to M. Pharm.
So, my advice would be go for M. Pharmacy. All the best.