Participants may be in a noisy environment. The lack of push to talk creates extra hassle of constantly having to mute, then unmute with the mouse control (even when participants are wearing headphones). Push to talk would solve this. All VoIP users should be able to assign their own PTT hotkey. When pressed it unmutes the mic, and when released, mutes it again.
Many voice chat programs like Ventrillo, Teamspeak, and Mumble, have PTT. This would be a valuable option for GoToMeeting, Webinar, and Training.