There are not a lot of words spoken about Spirit because of its nature. It includes and unifies Air, Fire, Water, and Earth. It is the source of the other four elements, and the four elements make up all that exists on the physical plane. It is beyond time and space. In other words, it is akin to the energy of the Divine. The upright pentagram represents, among other things, the four elements being governed by the fifth, Spirit.
Spirit is not normally used in spells, as it would "scatter" the magickal energy raised. For instance, say you wanted to do a spell to increase the passion in your relationship. The more symbols you have in your ritual that are associated with love and sex, the more powerful your spell will be; it helps to alter your consciousness. Therefore, you would want to invoke a love Goddess like Aphrodite for example, rather than use the generic term "Goddess". Likewise, it would be more efficient to use Fire rather than Spirit. The more specific, finely tuned spells are, the more successful they will be.