|MadSci Network: Biochemistry|
You're correct that the bicarbonate buffering system seems to be non-ideal for maintaining a pH of 7.4, but remember that the body produces lactic acid as a byproduct of respiration, so the excess capacity to buffer acids does serve a purpose. Because the body is constantly exhaling CO2, the system never reaches equilibrium, but rather holds bicarbonate and dissolved CO2 steady by the action of the kidneys and lungs.
You can try a Google search for blood+bicarbonate+buffer+pKa for more information.
Try the links in the MadSci Library for more information on Biochemistry.