As long as the dynamo can keep up with your power generation demands, as far as understand then there's no real incentive to swap it for an alternator?
If you're getting flat batteries due to running the car for long periods of time at low revs while drawing a lot of power, for example lights and wipers on while sat in big traffic queues then there's some potential gain with the alternator. Likewise if you want to fit a big sound system, or other high powered electronics that the dynamo will struggle to supply the power for?
Otherwise, might be a case of 'if it ain't broke, don't fix it'!