The purpose of the trial is to entice new customers, particularly ones who have never used FileMaker. Developer level people that want FMPA really are not new to FM and are not the target audience of the trial software. However, you can do a lot of testing with regular FMP if you want to see if it works and the only things you really are missing are the debugging tools. Most developers know they are going to have to delve in 12 at some point anyway, so you might as well go ahead and get over paying for the upgrade. FileMaker does offer annual maintenances and I use that so that I have a regularly scheduled annual payment and then know that any upgrades that come out I will receive without having to come up with an unscheduled payment.
Your conclusion that there is no such thing as FMPA trial is correct and it this is consistent with how it has worked in the past.