Yes but if it's thinner, it's better no?
Well at least that's what

think!
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But isn't the point of the all in one so you don't have to have seperate card readers etc?
I don't know about better functionally, but the general trend in TVs is thinner. And as much as I hate to admit it, even though it makes no difference whatsoever to performance, the thinner TVs look better. Now the iMac is not a TV, but whatever trend is driving thin as possible TVs seems to also apply here. I do like the way my iMac appears sitting on my desk at work, vs. the older ticker model that friends still have on theirs. It just looks nicer.
As to the point of all-in-ones, I'm not sure. The original Macintosh had a handle. I saw a guy riding down the street on a bike with a on iMac under his arm. I thought he might have stolen it, but then I didn't think anybody would be that brazen. So perhaps there's still some of that old Macintosh spirit alive in the design. The original Mac also had all of its ports on the rear, and had as much thought about what the back looked like sitting on an executives desk as the front. I know I appreciate how mine looks sitting on my desk to those who enter the room. As for those ports, the difference is today we pass around data on USB thumb drives, and SD cards, which should be a little easier to access than the rest. The thumb drives were easier to attach when the keyboards were wired, but the remote ports are gone with wireless. But maybe that's the point as well ... Apple wants you to transfer stuff wirelessly. Even the iPhone syncs wirelessly now -- though you still need a USB port to plug it into to charge it. So I tend to leave a Lightning cable plugged into my iMac ready for when I need it -- until I have to plug something else into the port. In reality I could plug my phone into any charger anywhere in the office, but that's not as convenient. Even so I end up with a bunch of cables hanging off the back of my Mac ready for when I need to use them. Hardly attractive or practical.
The SD card reader is perplexing regardless. Even the original Macintosh had floppy drive access on the front of the Mac until it was finally discontinued. And the SD card is the modern equivalent. The only practical way to access SD cards is with an external card reader placed in a more accessible location. I have he same problem with the Mac mini. Why include an SD card reader if it's not easily accessible? Frankly, I'd rather have an extra USB port if I've got to add an external card reader. There are card reader extenders, but this seems far worse than using a USB one:
@MCAsan has a point, if they're going to include a card reader, it should be far more universal than this old standard Apple continues to support. And maybe that's the point, that Apple's about to get rid of it, rather than keep up with changing standards. Hard to imagine as progressive Apple usually is that they've kept it around this long. But maybe that was their plan on the desktops, keep it there to make customers happy, but make it difficult enough to use so that customers come up with other ways to use SD cards anyway, and are prepared for when it's removed.
In general, desktops are the easiest to add adapters to, because they don't move. So back to your original question, it doesn't seem important to have card readers built into desktops. However, the question applies most dramatically to MacBooks, where I think the idea is that you don't have to carry around a bunch of dongles and accessories. And from all indications, it appears Apple is getting ready to drop it from their portables. and that seems like quite a loss to those who use the slot. Could so few customers actually use it?