i work at a major airport, and have worked for a few different airlines.
i never check a bag unless i have to. and would give the same advise to others. ive traveled for weeks with just a backpack.
airlines are not responsible for any lost items in checked baggage, because it states in the fine print "do not put electronics, laptops, valuebles etc etc"
TSA is a bloaded beauracracy. they hire just about anyone, someone that checks out clean with a 10 year background test only means they havent been caught. and there are still offenses that are acceptable.
all in all, the chances of someone stealing your stuff are higher the more set of hands handling it, meaning if your transfering flights or airlines. alot of the personel that transfers bags at airlines are contract companies, and they too dont have a strict hiring process, they also dont get paid much, most likely disgruntled and probably dont care if they lose their job.
of course the precentage arent high considering the traffic of thousands to hundreds of thousands of passengers a day.
not to mention the possibility of your bag missing your flight or getting lost.
oh and if your bag did miss the flight, many airlines are obligated to deliver your bag to your home free of charge. of course they never mention this because it costs them alot of money to do this.