this has been discussed before but people still get the wrong idea. you need to make a distinction between SPEAKER and AMP. COMBO AMP = AMP + SPEAKER
SPEAKER
alot of people mention that the speaker will be damaged because of distortion. but this will happen regardless of whether you use a guitar or a bass into a guitar amp. as long as the volume is too high, the sound will distort and the speaker will be damaged, period.
if you say that a bass will destroy a guitar amp, the same can be said about a guitar destroying a bass amp. the same can be said about a CD player lined into a guitar amp. the same can be said about a microphone plugged into a guitar amp. but who will do such things? that is the answer.
AMP
however, your bass will sound like shit if you plug it into a guitar amp. especially if the amp has chorus/reverb etc. this is because the amp is voiced for guitar. the same can be said about a guitar sounding like shit on a bass amp. that is unless that particular bass amp in question happens to be called the fender '59 bassman. that is a special example of a bass amp that sounds like shit on the bass.