The current model of pricing concurrent sessions is not very good for solutions with a very short and heavy load. You could argue that use PHP when you have that kind of load, but why?
Imagine that you could buy session in advance and pay 10 cents per session. One thousand sessions would cost $100. When the sessions where used up you would have to buy more sessions. This solution could have 100 concurrent users for a couple of hours and then go down to 0 users when the sessions down to zero.
This customer would never buy a 100 user concurrent licens that is unused for maybe 30 days and the have a heavy load for a couple of hours. With this solution FileMaker can be used in a lot more situations without threatening FileMakers need to make money.
It would even create more revenues for FileMaker since customers would pay in advance and come come back for more session (if and when) the sessions is down to zero.