They shouldn't... unless their job is to spread the gospel 100% of the time.. someone like Billy Graham would qualify for that.
But if you are preaching to the same congregation or two week after week, month after month... you are an elder and should work in the secular world for you and your families financial needs.
What do you believe about the scriptures that imply Christian leaders or elders should be paid? Their needs should be provided for at least. Paul ministered without charging for the gospel. But the worker is worthy of his hire.