Many years ago in college, (forgotten most of it) We were taught to attach the neg jump lead to the lift ring or other available neg point on the engine/chassis. But when charing to put the neg lead on the neg terminal on the battery.
Now that makes sense - when jump starting, you want the leads to be as close to the starter as possible, to remove any losses/high resistance joints, but when charging - you want to be as close (if not on) the battery for the same reasons. Hence my confusion - Also if connecting to change battery, I would have thought connecting to battery terminals was best, for the same reason.
When charging you do not use the car at all so the system is 12 volts.
When jumping its something to do with series and parallel. If you connect the jumping battery direct to vehicle battery the system becomes 24volts. If the jumping battery is earthed to different part of car it remains 12 volts.
nope only if you put the two batteries one after the other, ie + to +, - to + and - to -. If yu connect + to + and - to - they remain 12V.
The only reason I can see is that a connection to the injun is less likely to be knocked off, but if it aint a proper earthing point then you stand to loose data, not if connected to battery terminal, and it still doesnt remove the problem from the + terminal.

Actually, I can remember another reason for not connecting the neg to to battery - summat to do with the fact that IF there is a load on the battery, then when the second lead is connected, then there is a likelyhood of a spark when the old battery is flat, and batteries give off hydrogen (booom!), however I would imagine that that is less of a problem with modern sealed batteries. I cant see how it can destroy electrickeries if you are connecting a battery on to a battery.
Charging may be another problem as, I assume, there will be some form of ripple on the charging voltage, which might damage ECU's etc, however Alternators can also put out an AC signal and I am surprised if the electronics on board is so delicate that it cant cope with any noise on the 12V line.
I wait a good reason why this is a problem when linking another battery to change the original.