A question about PNG

Hi,

Actually, I'm not a guru at web design and concede that my experience in using  Adobe's Fireworks is not so great, but I don't understand one aspect -
Why does Fireworks use the PNG file format for its native editable files (the equivalent to .ai and .psd)? 
Don't you think that it causes a lot of problems for a great number of users?

Of course, professionals find an appropriate solution to solve this problem, but anyone can make a mistake and overwrite an important editable file.

What is your opinion?