The only disadvantage I can see is if it is expensive.

The advantages are:
1) It makes it easier to clean as you can remove the slipcover to clean it if needed.

2) You can change the slipcover to reflect different styles, perhaps changing it at the change of the season or changing it to match new furniture in the room.

3) It helps protect from minor damage such as sun bleaching or kitty claws.

Slipcovers are good for many reasons. If you want to try out a new look in a room, slipcovers are a perfect way to change a room in a simple or drastic way. They come in many different colors and patterns as well as fabric textures. If you want to test how you feel about the look of certain fabric, slipcovers are the way to go.

Another great advantage to slipcovers is they are removable and machine washable. They come in handy when there are children and pets in the household.

Slipcovers are also good for hiding any flaws in your furniture until it could be replaced.

Disadvantages to the slipcover are it could be somewhat difficult to find a slipcover to correctly fit on the piece of furniture you want to cover (such as a large couch with intricate arms). Some slipcovers are universal while others are more specifically made to fit certain shapes and sizes (and could be a little more expensive). If you want a custom made slipcover, that could be expensive as well.

A minor disadvantage to the slipcover is that, for some, it could more of a hassle. A lot of movement on and off the furniture could result in having to fix the slipcover back to the way it was. While being able to clean the slipcover is an advantage, taking it off and putting it back on could sometimes be exhausting.