If the 4 post a day limit was put in place to stop spamming, what's going to prevent it now that it's removed?
There is a bandwidth limit for the accounts, raising the limit requires more SP. A spammer could reach the limit fast, and with the HF, this limit won't be hardcoded but a witness parameter. If the witnesses see there are much spam, they could lower this limit (bandwidth / SP, I guess).
This isn't KISS principle my friend. Explain it to me like I'm a developmentally challenged 10 year old.