Most business plans are about achieving growth or change. The important thing is to make sure it is the type growth or change you want and that it is realistic and achievable. This requires careful consideration and I would always want to avoid it being one dimensional as a decision making process.
A Consultant who knows the market or simply has a good business mind can be of real value in this sense. I was involved in one process where there were three outside influencers all coming from different angles to help agree a vision and action plan for a firm. Their objectivity allowed them to tackle issues in a wholly different way to the owner managers. Quite simply they had no vested interest and could be honest and open with their opinions and expressed them freely.
Within law firms, plans for growth or change can become quite political and the managers need to make sure that they do not mix perception and reality as regards stakeholder opinion. A Consultant can be very helpful here as they will often have the ability to hold discussions with those stakeholders more easily and so unearth issues or concerns that may exist which if ignored could frustrate the best laid plans from becoming a reality.