My view of using a docked laptop as my main machine
It's perfectly possible to use a laptop as your main machine with a nice display, keyboard, mouse, and other peripherals. [...]
This is true. As Claire notes from personal experience, modern laptops can have displays, keyboards, and mice plugged into them. In fact, they can generally have exactly the same collection of keyboards, mice, and so on that you (or I) would be using with a desktop. I think the only possible problem here is exotic display setups that require equally exotic graphics cards to drive. A laptop will generally be less powerful in terms of CPU than a modern desktop (as I found when I started looking), and it may have cooling problems if run under heavy load for an extended period of time. But these are probably not problems for me in practice (regardless of what I'd like) and the 16 to 32 GB of RAM that you can get in a high end laptop these days is also not a problem.
(I'd like ECC in my 'desktop', but I've already given up on that too in practice.)
Still, I'm generally not interested, for two levels of reasons; there's a technical reason and then a broader emotional one. The technical reason is straightforward, namely that with laptops you only get one disk. I very strongly want mirrored disks, and until SSDs get much cheaper I need two pairs of disks in my serious desktop machines.
(Yes, really. I have over 750 GB of my own photos stored on my desktop, and that's only going to go up. Photography with RAW files adds up.)
There are workarounds for these issues; frequent backups, a NAS of some sort, and so on. But they add complication to the whole setup. Some of the complication would be good for me even with mirroring (such as frequent backups), but not all of it, and the situation even with workarounds is not quite as good as a desktop that takes four drives or more.
Which gets me to the broader emotional issue, which is that I have a fairly strong negative reaction to the idea of buying a laptop and then using it almost entirely as a CPU in a box. Because that's what a docked laptop is once you plug in a keyboard, a mouse, one or more external displays, a NAS, and so on, and use them instead of the laptop's own versions. This isn't a pragmatic, logical reaction, it's a visceral emotional one; it feels quite wrong to buy a laptop only in order to use it as a black (or silver) compute slab. Tentatively (since the situation hasn't come up), I think I wouldn't want to do this even if I had a reasonable amount of use for the laptop as a detached machine on its own. Right now, my gut would rather have two machines.
With that said, my gut may be flat out wrong here. To some degree I'm clinging to the familiar and so seeing all of its advantages and very few of its disadvantages; it's possible that if I switched to a single machine I moved around with, I'd come to love the setup and wonder how I'd ever managed to live with (eg) a fragmented working environment spread across N machines, all somewhat different from each other. A world where I moved a laptop between home and work would definitely be significantly different but I can't be sure it would be worse overall.
(With that said I see significant issues with aspects of this, like simply the process of moving the laptop around. You see, not only am I a bicycle commuter but I don't like backpacks and I not infrequently do things like go on after-work bike rides with my bicycle club. Hauling an expensive laptop around all the time in my bicycle panniers is not an appealing idea, and if the laptop is also my home machine I simply can't ever leave it behind at work.)
(I wrote a somewhat different take on this back in 2010, which didn't really spend much time on the 'laptop as just the system core' approach.)