Runway behind you IS useless because you can't use it. Simple as that.
V1 is V1. A longer runway will effect your performance calculations, but regardless, once you've hit V1 (just prior), your going flying (or should be)!
Another way of looking at it is this....
Specific Runway is 10,000 feet long. If your aircraft was quite light (Ferry flight for example) you''ll be airborne in less than 4000 feet. Once you've hit your computed V1, you fly. Now the runway
ahead of you in useless, since chopping the power (Most Boeing, Airbus and Douglas) and landing straight ahead is not an option.
Granted, during an RTO at heavier weights, that intersection departure may leave you wishing that you'd gone the full length. However - had you - you could have possibly computed a higher V1 speed anyways, so in the end, it's all pretty much the same.