ApparentSoft is a developer based in Israel, like me. They probably opted to use PayPal because PayPal recently opened up the ability to withdraw directly to an Israeli Bank account -- something which all the ohter entrants on this list cannot do (with the possible exception of 2CO, haven't looked at them).
I use them as a secondary account because some of my customers refuse to use PayPal, and 2CO really sucks compared to PayPal. They haven’t frozen my account, but their UI is crap, their fees are higher, there is delay before payment appears in my account, they keep a “reserve” when they do payouts, etc.
I've built merchant processor plugins for several different systems (2checkout, worldpay, alertpay) and NONE have as nice of an API as PayPal (both regular IPN and their PayPal Pro NVP API).
We've been selling through PayPal for 7 years now without issue, and we usually process from 2 to 3 hundred transactions per month.
I wonder if this is something of a best practice, to have two payment processors running live. Besides giving customers a choice, you also have the other one already up, running and accepting money if paypal or whoever freezes you.