Why do I feel the need to defend it? Because it's something I value and it's under attack. One could ask why you (rightly in my view) feel the need to campaign against changes in the PEL system, or others campaign to preserve historic buildings and ancient woodland. It's because the great mass of the population either doesn't care or doesn't know enough about an issue to make an informed decision, so it's up to those who do know and care to stand up and debate the issue.
To return to the argument (at the risk of boring the rest of the world into catatonia), if you accept that foxes are going to be killed, what is wrong with using another species to do it cleanly and quickly? From my own experience, it is a lot less cruel than the alternatives.