This would also create massive amounts of spam with people asking to be tipped for their shitty posts and people begging others who have been generous in the past
I don't want to be off-topic or controversial, but IMHO allowing paid signature campaigns already generates much more spam and shitty posts than the tipping incentive would. As DaRude says, tipping is a nice feature and practical use of Bitcoin, and abuse should be dealt with the usual way by moderators.