Why would you want a drop down vanity? What would be the benefit to you? How would you use it?

Depends on your taste and what you are using. If you have a bowl sink that sits on top of your vanity and you are short- you might want a drop down. If you vanity is a makeup one and the sink is in the regular counter- you might want a drop down and a seat. If you are on the taller side and your sink is inset, you might want the whole thing a bit on the higher side.

But a con would be if the vanity is a different height, you would need more counter cuts and cabinet heights on your order. That may matter or not to you.