Postive feedback systems destroy themselves.  (think of hooking the emitter to the base of a transistor)

Huh? That will simply give you an switched-off transistor... For forward biased operation a positive potential difference between base and emitter is required. One could of course achieve a reverse bias with that configuration, by keeping the collector at a lower potential. That would just give you a diode, though... As would hooking the collector to the base... In short, more than one transistor is needed for a positive feedback loop.

Let it be noted as well that some form of positive feedback can be very useful, and is used often in electronic circuits. The circuits don't usually blow up at all. Examples include astable multivibrators and circuits with input hysteresis, such as comparators and Schmitt triggers. These positive feedback arrangements provide useful benefits, such as voltage controlled frequencies for oscillators and noise immunity.

Positive feedback is only 'bad' when one does not want its effects.