There are actually multiple difficulties with tasers.
First as CloudSquallandZidane said, a taser won't always stop someone. However, the guy doesn't even have to be on drugs. When we went to our local jail for a school trip the officer there said anyone who signed a consent form could be tasered if they wished. He only let two of us do it.. so I didn't get a chance.
It connected the the shoulders rather then the cneter of the chest.. so it wasn't at it's strongest or anything. But really how likely are you to hit some one dead in the chest with a taser? Basically my freind kyle got it in the arm... no affect what so ever.. he didn't even twitch. We have other guys around here that can lift farmers electric fences up without major complications. They pick up the fence and let everyone else through.. the they crawl under or climb over. And while they are the minority.. there are still enough out there that our current tasers aren't a very good option.
However this isn't the only problem. Tasers only can shoot 15-20 or so feet.. and that may be military brand.. the brand on the market for citizens may be less. That is enough for most situations, but won't always help.. after all I believe even many of the weaker guns can easily out distant that.
What if you have two guys threatening you? With a normal firearm you at least have some what of a chance of disabling them.. but with a taser you don't.. they can only target one person. Even many rifles I have seen can shoot twice in a row... but not a taser.
Tasers are very limited, so are tranqs, and so are just about all the alternatives. There is a reason why Guns are considered the best defense. They can get the job done, though you need some training. Particually in hitting moving targets where you want to.
And for shoting someone stealing from me.. yeah I would .. but I would aim for the leg.