What if I'm optimizing for cost, not speed and reliability? My printer works just fine while I'm away, so I don't really care about a print taking an hour or two, and plastic is bendy and forgiving enough that a bit of accuracy sacrificed is fine.

I'd argue that additive manufacturing via hobbyist FDM printing is a lot cheaper for one off designs than subtractive manufacturing of any form, because the consumable cost is limited to utilized filament and a tiny amount of printer maintenance rather than huge amounts of waste combined with expensive, finicky tool bits and the same kinds of maintenance.

I do agree that for items like a Raspberry Pi case which could be mass-produced via molding to much better tolerance at a low cost, 3D printing might be silly, but for truly one-off, low accuracy consumer parts I don't think 3D printing can be beat.

I personally have also not had much problem getting square, true, dimensionally accurate prints with a crappy printer (Solidoodle 2). The most deflection I've ever seen is about .8mm across the 150x150mm build area. That isn't perfect and obviously won't work for precision equipment, but combined with ABS's flexibility and forgiving nature it's certainly good enough to make one-off enclosures, fasteners, buttons and knobs, adapters, and other "glue" parts, which is most of my use for 3D printing.

I will say that 3D printers are hard to set up, and that's probably the source of a lot of the warped cases you've seen. CNC mills and lathes are just as hard to set up, though - you've just generally paid the vendor tons of money to do it for you.