Unless you are aiming for loadsa work pruning and a constant algae battle then there is no need for 3 or 4WPG.
1WPG will grow a lot of plants. 2WPG will grow almost everything.
Advantages:
Costs less in replacement bulbs.
Uses less electricity.
Less items to go wrong.
Plants will grow more slowly.
Algae will grow more slowly (until you get used to the balance of ferts, CO2 etc.)
Lower light means more room for error.
Needs less ferts
NEeds less CO2 (if any)
This list could go on forever.
Disadvantages:
Can't think of any
Andy