If you're buying a laptop that ships with Linux (I'm looking at the Dell Precision line) they ship with the proprietary drivers installed. The OOBE experience from the manufacturer isn't the same as a vanilla install. I can't speak for the XPS 13, but I can say that my Precision M4500 runs flawless with the Quadro GPU.

Can't change the RAM - since moving to the Retina displays RAM is soldered to the board.

Not trolling - you can't upgrade the RAM (soldered, and the top option is 16GB anyway) and you're limited in SSD choices.

Last MBP I saw the insides of (a Retina-screen one, mind you) the SSD was the only upgradable part.

The other big thing the Precision line has over the MBP line (and for that matter, over just about any laptop on the market right now) is NOT having that damned chicklet keyboard.

Replaceable parts, and better GPUs. Next.

I can give a couple of reasons:
Better hardware options (Quadro vs Geforce)
Upgradable hardware - decide you need a bigger SSD or more RAM later? No can do on a MBP.

Most people that work on Laptops spend a lot more then 1K on a system. For a work laptop 1.8-2.5K isn't unreasonable. And they usually last a long time ( >5years ) before they quit working.

The one thing that a Linux laptop (say a Dell Precision like the OP mentioned) has over the Macbook is better GPU options - I can get a Quadro card instead of GeForce

There is, but it is extremely slow and clunky. Either SGI had some sort of magic they put in to their Firefox build (very possible) that people didn't figure out for 3, or 3 supports a lot of slowness that isn't in the older build. Since I just upgraded my Octane (upped the RAM to 1280MB from 384MB and the CPU from a 250MHz R10K to a 300MHz R12K) I intend to play with it again.

And relevant to this thread, the string for Firefox 3 is:

"Mozilla/5.0; U; IRIX64 IP30; en-US; rv:1.9.0.19) Gecko/2013020113 Firefox/3.0.19"

My string shows what I'm using anyway....

"Mozilla/5.0 (X11; U; IRIX64 IP30; en-US; rv:1.8.1.25pre) Gecko/20121112 Firefox/2.0.0.22pre"

Personally, I like the older Apple Pro Keyboard better then the current ones. And I'm still using a Sun Type 6 I replaced my old Apple keyboard with.

Microsoft Keyboards are pretty nice across the board. And I love the mice even more. At work I'm using a Sun Type V keyboard, which I love, with a Wireless Intellimouse.

The default option, last I looked, switched to the Windows 8 All Programs list instead of the Windows 6 style menu. The Windows 6 style menu, found in earlier builds can be switched back to via PowerShell. Not sure if they updated the default menu again.

You can, via PowerShell, change it back. I will be pissed if they take it out completely.