Just like it is not safe to hyperventilate, getting the CO2 balance to the other extreme is not secure either. High CO2 content influences the binding of O2 to hemoglobin, and so it is capable to transfer much less oxygen to the brain and other organs and tissue. This can become critical in depth, where the high CO2 level in your blood can cause blackout - that's even more dangerous than SWB, since you may be out of reach for you buddies or anywhere else who could possibly save your life if you blacked out on the surface or close to.
Eric Fattah wrote about such case for example in this thread:
http://forums.deeperblue.net/showthread.php?t=68233
Besides it, high CO2 level makes you also more suspectible to narcosis and bents. Read more about it here:
http://scuba-doc.com/CO2acclim.pdf