Also, what is the purpose behind decaying spent points?
The idea behind that model is to show a "what have you done for me lately" representation of points. You don't necessarily care that a player was highly productive six months ago, but you do care that he or she has been productive within the last month.
Decaying both earned and spent points effectively fades the past to focus on more recent contributions and deductions.
If only earnings were decayed, you would get disproportionate totals that would actually disadvantage your longer-standing members versus newer or spendthrift ones.