Some people eat pits in the idea that eating every part of the plant will give you a more complete spectrum of nutrients. Boutenko calls it the "macrobiotic principle", although I don't know how much it has to do with macrobiotics.
But this isn't a good idea. Many plant's survival strategies include deterring herbivores from eating the seeds, usually through producing phytotoxins of some sort. That's just their defense strategy. Otherwise, their seeds would not survive. Many members of the honeysuckle and roseacea family are like this. Some parts of the plants are edible, other are toxic.

here is what Victoria boutanko has to say:

Avocado Pit in Green Smoothies

Avocado

Last summer at the Raw & Living Spirit Retreat, my friend Gabrielle Chavez introduced me to a green smoothie made with an avocado pit blended as a part of its recipe, Since then I have been experimenting with adding avocado seeds to my smoothies, and now I use them frequently. Here is some information that I gathered about the nutritional properties of avocado seed.

* The total antioxidant capacity and phenolic content of edible portions and seeds of avocado, jackfruit, longan, mango and tamarind were studied. … The seeds showed a much higher antioxidant activity … than the edible portions. The contribution of all the fruit seed fractions to the total antioxidant activity … was always more than 70%. Source

* “Avocado seed contains relatively low amounts of calcium and magnesium, somewhat higher amounts of phosphorus, and high amounts of potassium.” Source

* The seed of the avocado contains tannins, and these make it bitter or "puckery" to the taste. And some of the tannins can be mildly toxic, though an individual would have to eat a lot to see any effects, unless that individual had a food allergy. Source
* Dr. Tom Wu: When I eat the avocado I always eat the seed. It is packed with very good nutrients. It has the life force. It is a very high Qi food! It’s also the highest in soluble fiber. This soluble fiber binds to the fat and excess cholesterol. Then we can lower cholesterol and improve heart function naturally. We can improve the blood circulation by pulling out all the fatty deposits in our circulatory system with the soluble fiber of the avocado seed. Soluble fiber is very difficult to get in our diet. Oatmeal has some, but it cannot compare with the avocado seed. Any heart disease patient must eat the avocado including the seed. Source

Based on these and other facts, I concluded that adding avocado seed in small proportion can be beneficial for nutritional needs. Also I have noticed that it helps to make a consistency of some smoothies less frothy, as in case with green smoothie that contains apples, cucumbers, or pineapples. I like to add ½ half avocado seed per 32 oz of most of my green smoothies; then it doesn’t affect the taste of smoothie but makes it a little thicker and provides additional nutritional benefits. Here is a sample recipe.

Green Smoothie Sweet and Sour Delight


4 Fuji apples, sliced
2 medium lemons without peel
1 banana, peeled
1 bunch parsley
1 cup spinach
1 stalk celery
Small piece ginger
½ avocado seed

Yields: 32 oz