Mark, I think we are closing in on the nut of this issue. When praying privately, we are not tempted to form our words in a way that impresses others and inflates our spiritual status. Period. If praying before others, that temptation increases (perhaps unconsciously) with the amount of experience we have with such exposure. (Maybe public prayer should be used rarely, or should always include confessing our sins before others. ) If I'm right, the question becomes, Is it possible to counter the well-established (Pharisaic) custom across our churches? Is it desirable?
I've been mulling through this question quite a bit. I don't think the evangelical community has a very robust theology of corporate prayer, both spoken and silent, something for us to think about. I think about the Asian style I encounter in which everyone prayer out loud at the same time- no worries about what the other person is saying but I often can't think straight.
It is important to think through why we are doing something- why do we pray out loud or silently? I don't know that doing it less is the solution, for example if something that is honorable to God is also a vulnerability to sin should we not do it? If some one reads scripture out loud but is given to pride should it be done less? The issue goes to motive. If we don't do it to glorify God why are we doing it? It can be that we are doing has become idolatry.