Open-Air Preaching is Optional?

Marvin_Newman_Untitled_soap_box_preacher_2787_102

Most of the pastors and preachers I know believe that open-air preaching is optional at best, and some go so far to say it's unhelpful and passé

What if it's NOT optional? What if it's expected? What if it should be normal and natural for preachers? 

How would you respond if I said God expects every man called to fill a pulpit is also to fill the open-air, the marketplace, the fields, the empty lots, etc, with the preaching of the Gospel of Jesus Christ? (And I don't just mean through personal evangelism, but through public proclamation.) If you think it is optional, can you provide any Scriptural argument for that? I honestly want to know if you disagree and what you base your position on.