For me anyway it's simple: I go to the talk because I think it's going to be interesting. If it turns out to be boring, but I feel like the speaker is making a real effort, I will stay and try to multitask my way through it, partly just in case something interesting comes up but mostly out of respect for the speaker... who after all is making a real effort.

If I feel like the speaker is just lazy then I'll get up and go somewhere else.

Note that this only works above a certain size: if it's happening in a conference room I will at least pretend to take notes on paper instead of reading email / HN / bugs / etc.

I'm not really sure whether this makes me more considerate, or more duplicitous, or maybe both.