Over at the GeoffRe(y)port, there is an interesting post on the unintended consequences of professional clergy. As I understand it, the problem is not so much with the elder(s) being supported by the church, as it is with what happens when it becomes accepted that all elders are to be fully supported by the church, and that if you aren’t fully supported, something must be wrong. In the early church, when tithing and giving was at its peak, a pastor being completely supported was the exception rather than the rule. Anyway, go read the article and join the discussion there.
One thing that I’ll add here that didn’t make it into my original comment is that a professional fully supported clergy also results in a situation similar to that of institutional academia, in that you can frequently end up with people who have no experience in the real world. Early church elder’s had to be competent men; they were not fully supported by the church, they had families and therefore were required to have productive incomes and they also had to have time to tend to the church and to study the Word. Today, in certain circles, the ministry can be a lucrative and cushy career path.