Since a large proportion of the people I work with every practice session are first-day or third-day beginners, I answered 'somewhat often' to the poll question. Most of them are just too confused to function by simply observing - and 1) I am too impatient to stand waiting for 10 minutes while somebody's trying and failing to break my arm 2) there is no point in practicing if they don't learn anything at all and just feel frustrated.
On the other hand, there's some more senior people in our dojo who just talk, talk and talk (read, correct, correct and correct) when you're practicing with them. That IS tiring, even if some of their comments may be helpful sometimes.
In any case, in all except the simplest cases, I only understand things when I _experience_ them, not when I am told about them.