Why not connect negative jumper cable to a bad batter? I've always jumped my car ('99 Toyota Solara) connecting both cables, but my battery just died and I was researching car jumping, and everyone says to NOT connect the negative cable to the bad battery, and hook it on another metal piece of the car instead. 

Well, I just tried that, and my car would NOT start. Frustrated, I connected it to the negative side of the battery just like I always have , and the car started fine!

So why do mechanics recommend not attaching the negative side, and why did mine only work when I did that?