I was hoping that someone more knowledgeable would answer your question, but since noone has let me take a stab at it.
Those who defend paying for inclusion in Ink make two points:
1. Paid inclusion will get you in immediately, free inclusion takes an unspecified amount of time.
2. Paid inclusion guarantees spidering every 48 hours, so you can make changes to your site and observe the effect on the SERPS within two days, this is almost "real time" compared to the free crawl cycle.
I have never paid for inclusion, but I think the above represents the case that several members here have made.