Frankly I'm nit sure why people have such a hard time grasping why the iMac is such a terrible solution for many tasks. Marvin especially comes up with some fairly tortured defense for the iMac that take all of a milli second for somebody to start laughing at.
The problem is there are many many tasks that computers are put to work on where the iMac can't even play the game. For example you can't plug in a high performance frame grabber card. You can't do a storage upgrade easily nor supplement the purchased storage internally (I don't even want to hear about external drives here as the are non starters). Not to mention the IMac is huge which actually makes it difficult to implement in many settings , think a lab here not some consumers cheap ass Ikea desk.
People tend not to come up with realistic scenarios about why the iMac is so bad. It's always about not being able to plug in some obscure peripheral, which there's probably a Thunderbolt solution for, or it gets too hot and shuts down, which it doesn't, or it's too big despite being smaller than a Mac Pro and not significantly larger than any other display, or it needs a desktop CPU, which it has, or it needs 64GB of RAM, which it doesn't, or it needs to be able to house a 250W $1000 Titan GPU to play World of Warcraft (because presumably 90FPS on Ultra at native resolution with the 680MX isn't fast enough).
The motivation is simply to get the fastest spec hardware for the best price and it's only marginally better than what Apple currently offers at the same price points. It's really just boils down to 'I don't want to buy an iMac' and that's fine but Apple isn't going to change strategy after this many years when they have taken the bulk of the premium computer market. Their business is where they want it to be. If that's not where you want their business to be, buy something else and if enough people do it, they'll change their business model.