One will be exposing a very vulnerable backdoor to his pc because today, the player can serve ads, the next day, without needing our approval, because you already installed the player on your machine, the player can serve malicious codes, which could be anything, like stealing credit cards numbers, and passwords and you will not even be aware of it, since it does it silently. It is not like a virus that causes blue screen or wipe out your hard disk, but more like a trojan or worm, that does damage by eavesdropping and making oyur pc a zombie or part of a botnet when it chooses and anti-virus software may not be able to detect such codes, if the sophisticated ones run as a rootkit.
Dun know about you, but I will not infect my own pc with such a backdoor, especially not so as to watch some videos.