Comparison is not reason.
"DIgikam is light years ahead of everything else on Linux, but it's still light years behind Lightroom in terms of useability."
Agreed.
"Lightroom opens my RAW images and gets the colours more or less spot on." ... "Digikam opens my RAW images in about 2 minutes per image, gets the colours completely wrong, and then many settings have to be applied using a 'Try' button which takes a very long time."
As of colours, LR comes packaged with many cameras ICM by default. Digikam expects you supply your own camera ICM. Either you're lucky to find a good one floating on internet, either you need to build one yourself with Argyll. Don't blame on Digikam what results from commercial bonds between Apple and camera manufacturers. It's a sad state of affairs cameras are not provided with accurate ICM like they used to be.
Speed : agreed, digikam is slow, and the preview button is annoying. Hope this will be addressed soon.
"And that's without enabling colour management, which slows everything down even more"
C'mon, is your computer still steam powered ? I've got a 2007 AMD X2-64 3800+, and there's no measurable difference induced by colour management.
"And where's the non-destructive editing?"
In next release along with face detection for auto-tagging of persons. Or so says the roadmap. No excuse, I know. I hope they really make it work.
"to regard it as an alternative to Lightroom is to demonstrate that you prefer sitting in front of a computer watching progress bars to actually taking photos."
Allow at least some of us to have principles we stand for. All my personal used softwares are GPL or free licensed. Until Apple changes a couple of policies, I won't have any at home. But then again, I'm not a professional, just a conscious amateur. I take around 10 - 20 pics a day, and I work on 1 or 2 at best, which I consider 'good enough'. Others are archived as is.
For a pro, LR is certainly the way to go. For others, it depends on the load.