I believe a lot of it depends on what type of game it is. There have been some really ambitious projects come to the preview program that have shown it to be worthwhile. On the other hand there have been a few that are not so ambitious and lack depth. The good thing is that every game has a trial to help someone decide if they want to spend their money on the project. Elite Dangerous was one of the first titles on the preview program and it has flourished into a great game that is continuing to grow long after it officially released. Then there is Ark, The Long Dark, Subnautica, these are ambitious projects that can take years, and more money than what the developers have to complete their vision. In that respect a person can try the free trial, if the promise of the game interests them then buying the game for a reasonably reduced price doesn't seem unfair.

The preview programs do seem to get abused a lot. Ark comes to mind specifically, with a DLC pack out before the game has even left preview. There need to be better rules in place to prevent the abuse of some developers from getting out of hand.

I don't care if you like it or not... it's giving gamers a choice and that's the only thing that matters... if people want to take a chance on a game that's an individual choice who are we to make the choice for them?