A question from Yahoo! Answers:

When the four stroke engines are more economical than the two stroke type why are the latter still in use?

The relevance of this question is from the point of view of fuel shortage and the increasing cost.

Because they are much simpler; they do not have valves, camshafts, oil pumps, etc. Also, two-stroke engines produce more power, both per unit of displacement and per unit of engine’s weight. In many applications, this is more important than fuel economy.