I believe that a good sum of people out there are mixed to an extent. However, because that is the case, and only God knows what ethnicities we are throughout our families' specific tree, there should only be focus on our immediate ethnicity. That is the ethnicity of our parents and grandparents. That way is the easiest way to handle things. When you are filling out an application or test that asks for race, are you really going to think "Well, I am sure that my great, great, grandparents were black. That means I am mixed, so I will pick that [other] option." If you really consider breaking it down that way, then asking for race is pointless and we might as well pick the same option.
...that's just me.
With that said, I am half-Asian (Korean/Japanese) and half-Black. I doubt that my great, great grandchildren will have the luxury of that due to racial dilution. My children or grandchildren more than likely will marry someone of "full" ethnicity, so my ethnicity won't matter.