The lesser sabbats, notably those that change the seasons and which are more linked in with the nature based religions such as Wicca are Yule, Litha, Ostara and Mabon. Mabon is the most controversial in terms of being the modern addition to the group. Many Traditional Witches do not recognise Mabon.
Sabbats play different parts in different traditions and some are more important to individuals than others. Some witches do not resonate with a particular sabbat and may choose not to acknowledge it at all. Others may prefer to focus primarily on the Greater sabbats. It is also important to note that there need not necessarily be any consistency in practise. One year it may feel absolutely right to celebrate Beltane, the following year it may come and pass without celebration.
Its a matter of personal preference among witches. The only right way is what feels right to the individual witch. So in answer to your question, no, not all witches follow the sabbats, some celebrate all of them, some some of them and some none of them. Contrary things witches!
Image http://www.deviantart.com/art/Wheel-of-the-Year-13238843 (Morgandria)