The line has to be connected to the exchange though for ADSL to work and connected to BT equipment, except in the case of LLU.
Even though you're not using the voice part of the frequencies you are still using their network and will depend on BT to maintain it and come out and fix the thing when there are faults. Someone has to pay for it. Okay, you could have the ISPs pay it instead but BT will still charge a line rental to the ISP who will just pass on the costs.
This is all separate to the "wholesale" stuff which is just the base ADSL price and nothing to do with the line cost.
Line costs are for the copper cable, regardless of what's actually sent down it (be it voice, ADSL, ISDN, etc).
I agree though that if you decided to not have a voice phone number it would be nice if BT offered a cut down rental that didn't include Option 1 and knocked a bit off for that part of the line rental that only related to voice specific functionality (though most of that's paid for in call costs).
Contractually there's probably some need to have a physical land line of some sort, and maybe for legal reasons (to be sure you're not some fraudster who's renting a line for ADSL and doesn't live at the address and has some mobile number as their 'contact' where they can't trace where you live... well, maybe something like that). Also, you need a line for emergency use and someone has to be paid to maintain that (these days, if you don't pay the bills you can't even dial 999)
Personally I'm not fussed. I don't use NTL TV so I'm not forced to have an NTL line and a BT line is the same cost to me as an NTL line, so I'm happy to switch to BT to get ADSL and still end up paying less for broadband and line rental combined than I am with NTL.
*If* I can get it that is... we shall see as it may end up getting rejected
