EAPOL exchanges are also used to renew the temporal keys. The new keys are installed on the Supplicant after it sends 4/4, and are installed on the Authenticator when it receives 4/4[1]. If Wireshark must handle rekeying correctly, it must only use the keys after reading the 4/4 packet in the frame, because packets may still be flowing during the rekeying (even in case where they should not, because of buffering)
For the first 4WHS, not waiting for 4/4 is possible, but it's perfectly understandable that they were just lazy to implement it. 3/4 is still necessary as it contains group keys (even if you are not interested in them, but know that you will not see ARP requests from the AP or a client fromfor which you didn't have ano part of its 4WHS) and management keys. You may skip 3/4 too, but then you have no confirmation that the exchange was successful, because 3/4 is used to verify that the Authenticator knows the PMK.
[1] Note that with the current scheme, if the 4/4 message is lost, then the supplicant started using the new key, and the authenticator still uses the old key, and resending 3/4 encrypted with the old key will not help. This problem, among many others with WPA2, is addressed in the latest 802.11 2012 standard by keeping two keys in parallel.