Here's how I see it:
Programs take more risk on PPS, while affiliates have the lower risk. That is why affiliates generally earn less money on PPS than with Revshare.
Let's assume the site's average rebill is 3 months. Commonly, the program might offer up 50-60% of three months membership as the high-end of their PPS scale. Some might only offer 30-40% though, depending on their expenses (as well as greed lol).
For Revshare, they probably offer something in the 40-70% range. This means that on average, a Revshare affiliate WILL earn more than a PPS affiliate. The program has a LOT less risk on Revshare since they will only payout money earned already, while on PPS they take a risk that the person's signups do rebill at their average.
If the program actively monitors their retention rate and revenues, they should be in fine shape offering both Revshare and PPs. I think the only ones who find themselves in trouble, are those who offer more than hey can afford to be 'competitive', or those who set their PPS rates based on variable revenue (such as upsells).
In conclusion, test out the program yourself. If you start on PPS and notice the site does rebill well, start switching some traffic over to Revshare.
That's my comments, I don't think I'm saying anything that isn't already obvious though
