The full exhale is called "purge". It washes out some CO2 from your lungs, hence it is a mild hyperventilation. Most freedivers I know do it. Some divers, who prefer diving on normal level of CO2, try avoiding it. There is for example the following thread discussing advantages, risks, and disatvantages of hyperventilation, so reading it can help you understanding the impact.
http://forums.deeperblue.net/freediv...ntilation.html