There's a fundamental difference between shareware / free trial apps and apps that require constant in-app purchases to... well... play. Doom and others use the free trial model, which is what we used to call shareware (before the internet, they wanted people to share the free trial with each other to inspire them to then go buy the full version). As long as these are properly listed as such, there's nothing sleazy about them at all. And it's fine to use an in-app purchase to go from trial to full version. Again, as long as its clear to the user that this is the way it works.
Games that have in-app purchases run the spectrum. Some use in-app purchases to do customizations and other fun things that don't affect gameplay. Then there are those like Dungeon Keeper which are basically unplayable without in-app purchases. Watch the video I linked above for details on why. You have to pay over and over and over just to keep the game playable.
This is very different from a shareware/free trial model. You pay ONE TIME and you own the whole game. It's a classic commercial game scenario except they give you a taste of it... a trial... for free. It's the same thing with 'lite' versions of commercial software that are also free.
The in-app purchasing model is more akin to getting a free version of an FTP client that only allowed you to upload software at 10KB/s. You can then pay $0.99 to upload that one file at full speed. But, your very next upload would again be 10KB/s unless you also paid $0.99 to upload that one at full speed. But wait, for an even better value, you can buy a 20-pack of full-speed uploads for only $9.99 or a 100-pack of full-speed uploads for only $29.99. Once those run out, of course, you're back to 10KB/s. Unless you spend even more money.
Now, it seems silly with the FTP client example above. Who would ever use that software? But this is exactly how many of the free to play games work. And this is the way a lot of people coming in to gaming for the first time think games work.