1) Reps are definitely not useless. Not even when they hit 100 million. This would not cease to be a subjective issue!
2) Negative repping would be a very dangerous weapon that would definitely be prone to abuse.
3) The official argument for resetting appears to be to avoid cluttering the profile area. If this were the case, a technical solution would have been sufficient: Make crowns more "valuable" (=> One crown for every 50 million reputation points). The profile area would have stayed compact.
4) Does anyone, by the way, know how many reputation points yield a crown in the new system?