TiJay wrote: A video of an event is a video of an event and therefore better than nothing, surely...
Any video can be used in court but, for many cases, establishing that the video really relates to the case in question requires that the court be satisfied that the video was taken on a given date and time.
Prosecutors prefer videos that have time/date codes because all you need to do is establish that the equipment's timing system is working correctly to establish the date and time. Without a date/time code you often need a witness to attest that the video was actually taken at the time and date claimed, and the credibility of that witness is likely to be critical to the courts willingness to give weight to the video evidence.
In the cases involving YouTube videos, that I have encountered the police have had to expend considerable effort to identify the location and time of the incident, and I would imagine that for every success there are probably a great many more cases where that was not possible.