Again a very extreme example but lets compare 100% haste with 100% crit. using only Shadowmend both would result in the same hps with the crit variation also gaining +100% hpm.
However, the 100% haste build makes twice the amount of casts and can do so on different ppl, leaving twice as many ppl with atonement. Meaning that SW:P will grant more ticks AND heal more ppl. Meaning that if you consider +x% haste equal to "+x% atonement count" Then spells like penance will grant the same HpM with both methods. The one will grant +x% more healing while the other will affect x% more ppl. SW:P actually achieves the higher HPM via haste than via crit.
I am not at all saying that haste gives the same or higher hpm (since the applying of atonment happens faster, it will also cost more) But haste does grant some hpm since there will be more atonements out. (This is what total meant with the static 15sec atonement duration)
Oh and just a little bonus, don't forget that mindbender actually also generates more mana with haste.