Maybe so. But none of that affects companies taking firm action against people if and where they do have evidence. It certainly isn't a basis for them not doing so, because that same logic would suggest that because the police can't catch and/or prosecute all burglars, they shouldn't bother trying to catch any.
I don't think anyone's suggested that they think these letters, or any court cases that follow, are going to magically make all piracy cease. But it may slow it down, and even if it doesn't, it'll at least generate some return for the companies that are losing out. And if it means that some pirates are paying compensation on behalf of others, oh well, crap happens. :d