Wouldn't it be nice if ignoring somebody prevented them automatically from commenting on your own threads? Ignoring on its own is not really that effective depending on what the goal is. If the goal is only to protect your brain from a user's bullshit, then it is effective. Otherwise it really isn't useful for anything.
I don't see any problem with it. You just don't watch what this person writes, so you're not care about where this person is writing in your topics or not. This is my view.