I don't think there is a perfect policy that can work for everyone here.
Not having a trademark leads to what VLC is now facing. That isn't an issue for Linux, GNOME, Qt, etc. because they aren't popular consumer products like VLC and Firefox. For those last two though, this is a major danger.
Having a trademark will always lead to friction at some level. It adds some limitations on what people can do with your code. But optimally this can be resolved in most cases (like how Ubuntu can ship branded Firefox).
So there isn't a perfect solution here. But I think that Linux and Qt do not need trademarks, while VLC and Firefox do.