BBC concerts aside, I've attended a lot of conferences and I've chaired a lot of conferences and I have to say, any chairman who doesn't get the audience to show their appreciation as a presenter is coming on stage isn't doing their job properly.
I think it's embarrassing and soul-destroying to be at an event where all you can hear is the speaker's footsteps across the stage as they head towards the podium. So "Please welcome..." is, in my view, just plain good manners.