I do not think you have to pay. We actually already pay a fee to have the app get some benefits/less ads, but unless they've just changed things drastically I feel like you'd just see more ads again or something.
If they do go to a pay-only model we will explore other alternatives when we migrate off Vbulletin next year, because that's just dumb.
Aha, yup, that's going to be the biggest drawback. I need to double check but we're also on an older plan for the boards that's not too much price-wise and used to totally eliminate ads, but they've gotten greedy and now it only supresses a limited number of them, and once that cap is reached starts showing them again. The newer plans are much more expensive and iirc don't even guarantee total ad freedom.
You at least got a free trail out of it. Enjoy the hike.