My main reason for wanting an AC-powered DMM is to be able to set up something on the bench and have a meter monitor it constantly, without having that consume the battery in a handheld meter, and without having the handheld automatically power off (to save the battery) after a period of no input. Almost any AC-powered meter would provide that, including the 3 that I named.
The other attraction of these meters is that they were all available locally. I was able to power each one on, see that the display worked, and check calibration on a few voltage ranges before buying them. And they were inexpensive enough that if they turned out to have problems, I didn't waste much money. The 8600A actually did end up having a 5 V regulator problem, which I fixed, but otherwise they all seem to be fully working. I am less comfortable buying something electronic from a random person on eBay without any chance to look at it in person first.
Although the Fluke 8600A and 8050A likely haven't been calibrated in 40 years, there are applications where their greater precision (4.5 digits) is useful for making differential measurements even if the absolute measurements are not within spec. I have access to a 8840A at work and I intend to check my Flukes against it, so then I will know how well they are calibrated (and the Fluke manuals have circuit diagrams and cal procedures, so I can adjust them if necessary). Also, in limited DC volts testing, all 3 of the meters agree within their respective accuracy limits, so either they are still in spec, or they've all drifted by about the same amount in the same direction.
Other replies have convinced me not to touch the calibration adjustments on the Instek meter unless I find it obviously wrong. At that point, the main chip datasheet and the simple circuit board should let me figure out which adjustments are involved. Maybe that day will never come.
The BM235 seems like a nice handheld, but it's still battery powered, with slightly worse basic DC accuracy than the two Fluke meters, 6000 count instead of 20000 count, and stupidly expensive in Canada. The Fluke 45 and 8840A both seem like good meters, but fully functional ones seem to sell for considerably more money on eBay and I haven't seen them for sale locally. Maybe once I'm retired I'll find that I use a bench meter frequently and I'll be willing to spend that kind of money, but at the moment I might use a bench DMM once a month if I'm lucky.