The likelihood of someone randomly seeing your bike and taking it are slim. This is something that generally has to be planned. That or the thiefs are not very bright. So unless you park it or frequent some place very often I would not put much thought into it. In the end, if a thief wants your bike he takes your bike, period. I don't care what you have on it.
Alarms are a waste of time. People don't pay attention to them anymore. The thief can not start the bike without the factory key. So by the time you figure out what is going on, the bike is on the back of a truck, wastefully wailing away!