From the users' point of view, the one thing missing with Apple Apps compared with Android ones is you can't get a refund if you don't like the App or if it doesn't do what you wanted - or even if it doesn't work.
A 'free' version of the App helps with that - but of course the 'free' one will be a cut-down version, which may not have the feature you want to test.
Perhaps a 'rental' of the full version might overcome this. You get the full version and try it out. If it does what you want then buy the full version.
On the other hand, it is not as if we are talking of hugely expensive software, like that on Windows systems. If the App cost a couple of pounds and the description looks as if it might do what you want, then give it a try. Even having said that, the Android Market lets you buy an App for a similar sum and still get a refund if you delete it from your system within a short time - so you really can try an App without risk.
If the App costs £1.99 to buy, how much would it cost to rent? I suppose this would add up over time but it might seem a lot of technical overhead for very tiny amounts. That sort of comment might explain why I am not as rich as Apple of course