[jacquesm]

Account sharing in a telco context is a bad thing all around. Which phone would you like to ring? How do you ensure the charges really are made by (and to) the right person? How will you protect against messages with important information landing with the wrong party?

Authentication in a telco context is a good thing, the fact that the web doesn't have it enabled a large number of applications to flourish, it also made some other things devilishly hard, or even almost impossible.