I have an automatic and it never happen to me. I say its very dangerous to have the engine stalled when driving. It happened my dad's Merc. before, I was going down a pretty steep hill and the engine stalled. Its not just the power steering, the power brakes also went and the stopping distance, I would say more than doubled. It a fortunate thing that there were no cars in front.
For the Merc., the dealer told us that it has a faulty engine management computer or something
and they change it and the problem was gone.