Well, the light was on because when you removed the cat you probably permanently disconnected the O2 sensor; so your mechanic simply removed the light bulb. The problem now is that light is supposed to also indicate if there are other problems. Typically the light is just a flag to tell you that you need to run a dignosis (for recent vehicles OBDII). Problems like missfire, ignition and many others may not be caught because of your configuartion.