Almost nobody saturates their bandwidth like that. Unless both the wardriver and you are continuously transferring large files, you're unlikely to notice any performance hit.
As well, your garden-variety wardriver is almost certainly not an authorized user of the ISP's resources, and hence is receiving a service he is not entitled to receive.
That's the real issue. It's similar to the Napster argument: for each person who puts 1000 albums on his laptop, a record company claims a $20,000 loss, because that's what it would have cost to buy all that music at retail prices. But that's not quite right, either, because most of the music available on the internet is clearly stolen, whereas many of the "promiscuous" wireless ports are intended for use by the public, and there's no way to distinguish public from private.
If I'm not home at the time, I'm unlikely to notice that you've come in and spent some time watching my TV - that doesn't make it okay to do, though.
But that's not quite right, either, because most of the music available on the internet is clearly stolen, whereas many of the "promiscuous" wireless ports are intended for use by the public, and there's no way to distinguish public from private.
Err on the side of caution, I suppose. The question is whether he reasonably believed that this WAP was intended to be freely available to the public. Opinions on that will vary, I imagine :)