My company follows the same practice as that of ETOPS. On oceanic sectors, and on SELCAL watch, headsets are removed and the speakers monitor Guard and Air-to-Air from the VHF boxes. Our aircraft are not fitted with handheld mikes.
In busy areas it is a dubious practice to rely on speakers. I can only vouch for our models of the B75/767, but if one pilot keys his mike for any purpose, incluiding cabin interphone or PA, it isolates the speakers. This is a potential hazzard if flying off-headset in a VHF environment.
We regularly hear ATC calling a flight several times for a frequency change, etc, without response, only to hear the said aircraft chirpily repy, some time later, expressing utter amazement that it had not received the previous calls. I often suspect that this is because both pilots are off-headset and one is engaged in treating the customers to a graphic description of the local scenery, or a meaningful discussion with the cabin crew.
If you fly an aircraft with this feature, make sure you go back on to headset before your colleague starts talking to the cabin or the company. Better still, keep the cans on when it's busy.