The problem with jumping a motorcycle battery from a car or truck isn't a voltage issue - it's an amperage issue. If the battery is extremely flat (won't even crank the bike, very dim headlights) the donating system can provide far too much amperage (in a very low battery, it can be seen by the donor as a massive load) and the resulting inrush current can exceed the capacity of the dead battery to absorb it. Next step - boom (and yes, as an electrician I have seen this happen, and had to fix the results).
This is why the donor vehicle shouldn't be running if it has a higher current availability than the recipient system (most car systems have 100+ amps available!). I carry jumper cables that are sized for a motorcycle, the smaller cables help to limit the amount of current flow available. If my battery goes so flat I have no cranking or headlights, I won't jump it - I'll either have the bike towed home or buy a replacement battery, once I fix the reason it's dead, of course!!