Do you remember how annoying it was to be unable to -rep a shitpost or +rep something hilarious because you'd already used it that day and had to "owe" them one?
Rep manipulation being against the rules I can understand because if allowed, many users can falsely represent themselves on their profiles (who actually looks at other people's profiles though?) or +rep their own posts in heated discussions to make out like they have friends who agree with them (which happens on YouTube all the time). The act of doing so speaks in volumes of how pathetic a person has to be to do so. It's not healthy behaviour.
But if getting negrepped is that big of a deal, what else do you suggest? Without much thought put into it, I would suggest limiting how often a single user can negrep another individual's posts. Maybe like 2-3 posts in the space of a day. I would say limit +reps too, but that won't discourage Rep manipulators with multiple alts so there'd be no point, another solution would be needed for that. Perhaps increasing the post limit required to rep people? Right now it's 15 posts if I recall, unless that was changed.
The only problem with current system is some users feel a reputation makes them "special" or "unwanted" in few cases certain users feel threatened by others with anonymous negative reputation post attacks.
My account > Settings > Notifications > Reputation > Dropdown to No > Save changes