It's not just a cosmetic issue. Any time a rabbits nails start to get too long, you run the risk of injury... I recently saw a rabbit with long nails that got one caught on a bit of carpet. Ripped the nail out at the base. Very upsetting to the owner and the rabbit. The nail will not grow back in and he could have broken the toe.
Another issue is if the rabbit's nails are too long, it causes the toes to twist as the rabbit hops about. Long term issues with this (especially with larger rabbits) is arthritis in the toes and front legs, or sore hocks (trying to compensate for the lack of balance).
Even on a natural floor (sand, dirt, stone, etc.) some rabbits nails will grow to long. It is best that you check them at least once a month and trim them if necessary. All rabbits should be used to being handled and having their feet touched. (In case of injury or other problems.)