Interesting topic.
If I could toss in my two cents:
I can never last long the first round--the vast majority of the SPs I've encountered are so nice it gets me quite excited. Perhaps it's a mental thing, but once I'm in there I have 2 minutes if I'm lucky. Only once have I had it where I couldn't pull off a second round in 30 mins.
This doesn't subtract from the rest of the arguments that suggest the intimacy, eroticism, companionship, or cuddling is enough to make up for any quantity of SOG. For some it might. For me, I couldn't say it any better than Carrie Moon: "I prefer to fuck". We all visit SPs for a variety of reasons.
So to look at it pragmatically, perhaps the "rate" system might be a slight misnomer for some and should be modified. In my opinion, 30 mins should be 30 mins. If I last 2 of them, it's logical that I'm entitled to the 28 left. I've left feeling swindled when this is not the case. The inverse is also true. Over 30 mins, one must pay to play. Perhaps the solution would be to indicate, as some have, "30 min/ 1 SOG= $x"
For the most part, most indepedent SPs that I've seen are more than willing to accomodate anyone who goes in the first 5 minutes.