PTT is required because of background noise. If you use speakers (please don't) when you are talking or not talking, everyone else in mumble hears their voice through your speakers and then through your mic.
If you use a headset, then a lot of times everyone needs to hear your keyboard, your mom in the background, the fan on your desk, etc. Today heard a mouth breating neckbeard COPD guy heavy breathing for like 30 minutes. It really gets on my nerves!