What irritates me are the pedestrians who FREAK OUT when I do it. Like I would be saying it planning to hit them!
Personally, I'm a jumpy guy. If my back is to the door of my office and someone speaks to me, I get startled. So hearing a voice behind me on a trail is likely to have the same effect. For some reason, a bell does not. I think it's also the case that for pedestrians who haven't spent much time on MUP, the directions that we call out can be confusing. We shout out "on your left," but their brains sometimes process that as "move left." It's inevitable that some of the walkers on any MUP in a crowded area are going to be out for their annual stroll and just aren't used to responding to a voiced warning.
I've seen people on one of the really crowded MUP here going around 30 mph on tri bikes. I've also seen roadies going almost as fast in pacelines. Neither seem to want to relinquish any speed.
What gets me are the ones who think that shouting out "on your left" gives them a license to pass anybody at any time, regardless of how safe it is. I can't count how many times I've been passing a pedestrian on the left when some speed demon shouts out "on your left" and passes within a couple of inches of me. I fear that someday I'm either going to hit the passer or be forced into the pedestrian. Part of using a MUP is that you sometimes have to slow down even if you'd rather not, and that there are times when it's just not safe to pass (like when there's an oncoming bike or pedestrian in the other lane). People who have an unyielding need for speed need to get off the trail.