Advantages:
Less frontal area, so less wind resistance, hence more top speed.
More room in the bow area of the boat
Aesthetics
Disadvantages:
Less buoyancy, should you have the misfortune to stuff the bow (not that I'd know anything about that
)
Possibly more expensive to produce, due to additional complexity of toob design?
Fashionable? Tapered toobs seem to have become less fashionable nowadays, apart from on larger RIBs.
Smaller inbuilt 'fender'