Sloping top tubes are great for shorter riders as was previously mentioned. Think about smaller bike frames for a moment. Basically everything is shorter. The only problem is, the wheels are still 700. So the only way to compensate is to screw up the geometry if you don't use a sloping top tube.

There are also other advantages. On an alluminum frame, a sloping top tube with a carbon seat post gives you more carbon for vibration dampening (if you believe in carbon's vibration dampening qualities). More seat post also allows you to put the bike in a work stand without screwing up your seat height setting. More seat post also allows you to put a seat bag on and still have room for a light underneath.

There's really no disadvantage to a sloping top tube. Some manufacturers offer them in fewer frame sizes, which is a shame. Straight top tubes were necessary for many years because lugged frames required them. I suppose this is why some people who like the traditional look are opposed to them.