It is a model which allows people to try before they buy, which also expands the audience and allows the game to spread. It is a good thing when implemented well.
As someone who hasn't bought a single in-app purchase (I might if Carcasonne add expansions), are these transferrable across devices like the apps are?
Another benefit of it is that it restores all in-app purchases for this app. Very helpful in case the application provides more than one purchase product.
I recall reading that this is a hard requirement for apps that offer in-app purchases.
In-app upgrades from free to full are exactly that, and generally seamless... What are you whining about exactly?
> are these transferrable across devices like the apps are?
Yes.