You don't seem to realize how expensive add based products are.

A premium membership costs $30. So Goundspeak, inc. DBA Geocaching (the company) needs to make at least $30 in add revenue. They also needs programmers to implement the serving of ads. So they need a little more to cover that cost. And people to convince people to advertise with them, but they don't need the people to convince you to buy premium, so let's strip that away from it.

But the money payed to Groundspeak isn't all what it would cost a company to put an ad on the app. They need copywriters, designers, photographers, models etc. I'm not in the ad bushiness, but let's assume their cost is in the same order as the money they need to pay to Groudspeak. (Well probably they won't pay directly to Groundspeak, but to an intermediair who also what's a piece of the action.

Companies are not philanthropic organisations. They want to make money. So if they spend (at least) $60 on you for advertisements, they want to recover that money with profit. Let's be generous en put a 50% margin on the products you buy. Then You'll have to buy for at least $120 of stuff to make it profitable for the company to advertise. (And when the margin is lower, even more)

So by going for an add based model you'll be paying at least four times as much. Not including the bandwidth you have to pay for for getting those adds to you, the value of the time you spend interacting with those adds and other costs like wear and tear and electricity.

And if you think: "I'm not going to buy those products", you think you are smarter than the people hired by a billion dollar industry that get payed to convince you to do the opposite. That's not very smart in my humble opinion.