What are the benefits and disadvantages of acrylic yarn versus pure wool?

It is generally cheaper than wool
It is moth proof
Generally, it can be machine washed and machine dried, but read the ball band first!
It is mainly non-allergenic

The benefits of pure wool are:

It is warmer than acrylic
It is less likely to pill than an acrylic, though the more expensive acrylics are also less prone to pill
It will last longer
It keeps it shape better

Some wools can be machine washed on a gentle cycle, but most if not all have to be dried flat. Some wools are 'itchy' but some wools, especially the Merino blends, are very soft.

You obviously have to select the yarn for the application. In general, buy the best yarn that you can afford. Given the amount of time you put into a project, you want it to last. Some people find that a wool/acrylic mixture is a good compromise. Some wool/acrylic mixtures can be machine washed and machine dried. Read the label!