Should I Buy a Spare DI Cassette

Well, that depends on what other spares you're going to carry around. Yes, the DI does have a high failure rate, but so does the serpentine belt, it's idler pulley, the AMM, (especially) the clutch master + slave cylinders, and a few other things.

If you intend to carry spares + tools for all of the above, then I'd say yes, definitely carry around a spare DI. Otherwise just treat it in the same way as you would any of the other items, and buy one when it actually fails.

Yes, true. But you have to remember that if you hang around a hospital for a couple of days you could very easily conclude that the vast majority of people are ill most of the time. Same is true of car newsgroups.

Presumably that was a discrete device, as opposed to one which bolts on top of the engine?

Relocating it might just be possible - I don't know, I haven't tried it. If you did relocate it, you'd need to use copper ignition leads, as opposed to high resistance carbon ones. This will present the same resistance to the DI, so have some chance of working. However, copper leads chuck out an awful lot of interference (which is why they aren't used much), and this may well interfere with other vehicle electronics.

May be worth a try if you're feeling brave. It's unlikely you'll fry anything, but you may get some odd behaviour.

There's also the issue of your car becoming an nuisance interference source.