If this were the case, there would be far fewer practicing Solitary Pagans out there. If you are performing a ritual in a cast circle, you only need to acknowledge the directions, and find some method of drawing their energy into your sacred space.
Some people "summon" this energy by precise methods of Calling the Quarters, some people light candles to mark off and acknowledge the energies... any number of methods can be used. Having people individually call the quarters is very useful when you are doing a group ritual and you want everyone to participate. But it certainly isn't necessary.
~Flame RavenHawk