parallel circuit: Providing that the breakage does not result in a short circuit the other bulbs will still light.
series circuit: If the breakage results in a short circuit through the bulb the other bulbs will light more brightly. If the breakage results in a breakage of the connection through the bulb then the other bulbs will not light.
Only if the marketplace demands it. Like everything else, "Supply and Demand."
Alternative energy
i dont know
Two reasons:
As a bonus, the weight for the fuel cell fuel reduces the weight of another required consumable: water. The "exhaust" of fuel cells is pure water which astronauts can drink.
A fuel cell is different from a battery cell in so far as reactants are constantly supplied to a fuel cell making it an open system whereas a battery cell is a closed system that stores the reactants within it. A fuel cell works as long as fuel is supplied to it whereas a battery cell requires regular replacements.