Because Mt Gox is only made of a few, we can know who they are relatively easily. They are not anonymous.
Yes they could still screw us quite easily, but you can avoid disaster if you follow a few rules -
- Don't put all you eggs in one basket (Don't trade only BTC, use several exchanges ?)
- Building trust progressively is your best bet
Which brings me to the point of the OP, people quite possibly are trading both, and there are many reasons why one would trade BTC.
I don't see at all why its so mind-blowing, you don't necessarily have to invest your life savings?
Finally, you can't have expectations of entrepreneurs in a new market to have bureaucratic certifications and be following non-existent regulations.
How do you propose an exchange should be set up for the Bitcoin market?