My opinion is this: enemas are absolutely necessary during a long Water Fast and in the refeeding period after a long water fast.

On shorter fasts, less than say 10 days, they're probably not necessary but will probably make you feel better (as well as helping to rid you of nasty toxins).

On a related note, I've recently come to see the importance of scrubbing the skin during a fast: my preference is Dr. Bronner's Pepermint Soap and a loofah. Made me feel soooo much better.

Sinclair isn't the most up-to-date of fasting resources, but I really like his book The fasting cure By Upton Sinclair. It is on google books for free. If you search for "enema" inside the book, there are a number of interesting points.