Really, I'm not a fan but it all comes down to safety & happiness for horse and rider...
So many of these "aids" seem very victorian & severe & i have the type of horse who will just not function if she's not allowed to be comfortable.
I also don't believe in the "when used by an experienced person" thing as I know so many novices/single horse owners who really pay attention to what their horse is telling them but those who turn around lots of horses, have bums of superglue and years of experience may MAKE the horse do something, if that makes sense.
However, I let my horse poke her nose out most of the time as long as the power's coming from behind so my opinion means nothing!!!