When building your own NAS and it has to be compact.

When building an HTPC

Irrelevant;
There are mATX cases that are just a bit bigger, but provide you with more slots for your drives, which I suppose is important?
https://www.rosewill.com/product/rosewill-fbm-05-dual-fan-micro-atx-mini-tower-computer-case/

HTPC:
Irrelevant again; 
Plenty of mATX cases that are compact enough to fit into surroundings. Cube-ish form factor cases won't take that much more space than ITX, as long as heigh fits, which it should:
https://pcpartpicker.com/product/YLDzK8/silverstone-case-sstsg11b

In both cases, irrelevant to whom?

I mean, both arguments are pretty irrelevant.
If you are building NAS, I imagine you want as much space inside as possible within external space restrictions.
If you are building HTPC, chances are you will be able to sacrifice 10 cm of space in one of directions. 

You can have high volume drives, in turn reducing the number of drives you would need for a NAS.

10 cm is not insignificant and you won't be utilizing the mATX board to it's full potential, imo

http://www.cooltek.de/en/jonsbo/c-series/126/c2 12.10L mATX, is that close enough?

Since when ITX boards have 4 RAM slots, or multiple PCIe x16 slots, or components of rather good quality?

ITX boards are farm more restricting than needed. Want more memory? Buy new kit.
Want to expand your storage? Buy big capacity, expensive drives. Want to have good cooling without having to go AIO route? You're shit out of luck.

If you keep your build on the table with no height restrictions, the case can be 1 meter tall, it won't really matter. If you keep it in cupboard, then yes, every milimeter counts

Don't think I hate just ITX cases. Something like this makes little sense to me as well, althought fully silent (if you opt in for SSD storage only) build does sounds attractive:
https://streacom.com/products/fc5-alpha-fanless-chassis/

Okay, what's the price difference between 8x2 and 4x2 kits? 

Multiple PCIe sockets: 
https://pcpartpicker.com/products/motherboard/#f=2&l=1000&s=33&sort=price&page=1

My build is on the desk, takes barely any space, because Meshify C is not wide and long. 

i only need one picture:

Fujitsu Scaleo C with new internals. Only possible with ITX

That's a nice CD player, albeit quite big

Let's say you need 16GB for now, fine, you get 2x8 kit.

But what if you need another 16GB 2 years later due to crappy Windows? You have to get rid of your existing 16GB kit and buy a new 32GB kit. If your motherboard had 4 slots, you could simly get another 16GB kit.


P.S. Damn, I really don't think this case was a success:
https://www.nzxt.com/products/manta-matte-black
 

The problem with most itx cases are that they are just as big as matx, which is why you might think that itx is pointless.

But if you get a truly itx case then it is a whole different story.

Like when you get a case that is just as big as the motherboard:

(yes that fan is the stock cpu cooler fan)

... That example was just for the sake of argument.
 

Every form factor has it's pros and cons with their own purposes. 

It's okay to favor one over the other, but disregarding the ones you don't like as pointless and irrelevant isn't a reasonable thing to do, imo. 

But I am still yet to see a reasonable argument for ITX form factor.

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There are mATX cases that are just a bit bigger, but provide you with more slots for your drives, which I suppose is important?

That's a pretty piss poor example of more slots for drives. The LIAN LI PC-Q26 was a Mini-ITX case that could hold 10 hard drives.

Quote

But what if you need another 16GB 2 years later due to crappy Windows? You have to get rid of your existing 16GB kit and buy a new 32GB kit. If your motherboard had 4 slots, you could simly get another 16GB kit.

If you look into the pro-sumer server market you could buy the ASRock Rack C2750D4I or it's little brother the 2550. They use a BGA Avoton 8 & 4 core CPU respectively and each come with 12 sata ports and 4 DIMM slots. It's a great little board for a OP NAS.

There are also socketed Mini-ITX boards that support denser memory or even use SO-DIMM modules. If you have 16GB and need more but you occupied all your slots already then that's just poor planning on your part.