The purpose for Isaiah and his family is stated clearly in chapter 8:  "To be a sign and wonder unto Israel."  But just how far was he willing to go?  Oh yes, he said, "Here am I LORD, send me."  But did he mean it?

"At the same time spake the LORD unto Isaiah the son of Amoz, saying, Go and loose the sackloth from off thy loins, and put off thy shoe from thy foot.  And he did so, walking naked and barefoot.  And the LORD said, Like as my servant Isaiah hath walked naked and barefoot three years for a sign and wonder upon Egypt and upon Ethiopia..."

Asking to walk around naked for three years answers the question!  He was the servant that God used to carry the "Burdens" to the nations...but he had to do it with the above stipulation.  Shameful and unpious was this sort of request, especially for a Jew and prophet, yet Isaiah obeyed and delivered the message the way the LORD wanted it.  His life was not meant to be held in high 'dignity' but as a "sign" and as a "wonder". 

He is asking the Church to have the same surrender.  He has given us, perhaps, easier stipulations to follow than to run around naked, yet we still have a hard time obeying.  He has told us to forsake all.  He has told us to suffer.  He has told us to die.  He has told us to be the lowliest of servants.  He has told us to go into all the world and preach.  "Here am I LORD send me," is not enough to make you a servant of God.  Realize that your life is not so YOU can do something for Him.  The purpose of your life is to be a "sign" and a "wonder" unto the world and if that means God wants you to preach naked, then you ought fulfill your purpose.