I have no problem with street preaching - although I'm not convinced that it is effective as practiced today.
There are various ways that it is practiced today. Which approach are you referring to specifically?
I am opposed to assaulting captive audiences on buses and trains, however, as seems to be the practice of some group operating in Dallas these days. If you want to be a martyr, don't do it where I might end up as collateral damage.
I'm not sure how that amounts to "assault," but I tend to agree with you. I would quickly add, though, that I wouldn't throw out the baby with the bathwater. I think in that particular situation, where people can't get away, a better approach would be to hand out tracts to the people sitting down on the train and then try to start a one-on-one conversation with someone a little while later, kind of along the lines of, "Did you get a chance to read that leaflet? What did you think?"
But regarding effectiveness, it's a mistake from the outset to go about evaluating an outreach ministry on the basis of its results because we are unable to measure the results. How can we? How could we possibly evaluate what impact the gospel message had on any person in a crowd? We cannot. There's no way to know if even just one person in the crowd--someone you didn't even notice was there the whole time--went home deeply convicted of sin and was granted repentance unto life and faith in Christ.
I'm convinced that the great error that many evangelicals make lies in that very criterion of "effectiveness" they use to evaluate street preaching. For most evangelicals, effectiveness equates to visible, measurable results. If something does not provide a measurable return, they see it as unprofitable and try to come up with another approach.
If people are determined to use effectiveness as their criterion, though, here is something they should look for: glorification of God. The blog post by Phil Johnson made an excellent point: The goal of evangelism is not just to make converts. It is primarily to glorify God. For the purpose of glorifying God, public proclamation of the gospel that is faithful and accurate is always effective.