To me, the souls were not really important, what was more crucial was that we continuously ascended Reapers. The jump from a Wraith to a Phantom was not really difficult. And while creating Specters would take time, Phantoms were quite easy.
For that reason, I wanted to use the tax system not to generate funds but to strengthen my domain. I believe what I said at the time was.
"My battlefront tax will be paid in undead bodies. I have no use for souls. If you want to stay in my domain, you will fight. I will require seven bodies daily."
"Seven bodies? You mean Rank F's?"
"Yes. Anyone who isn't dying will kill seven bodies or answer to me. That is only 140 souls. Very cheap if you ask me."