Here's the deal: There are lots of different antibiotics because they work in different ways, on different kinds of bacteria. Some antibiotics don't do anything to certain kinds of bacteria (at all)...but the antibiotic will cause other problems, from upset stomach to diarrhea to deafness to death. When I pick a particular antibiotic it's based on what will work for the bacteria at issue, and based on what will work best for the patient: Some antibiotics can only be given intravenously, some have to be injected into muscles, some can only be given orally...because of the way that the antibiotics are absorbed and processed by the body (some drugs are converted into active forms in the body). We also have to consider how long the drug lasts, and how much drug is needed (based on the time it stays around, and how much concentration is needed to kill bacteria).

Some time ago I wrote a short list of antibiotics I recommend for people, you can read it here: http://www.awrm.org/ubb/ultimatebb.php?ubb=get_topic;f=9;t=000109

I submitted that to Rawles blog a few years back. Another Rawles blog reader, GeorgiaDoc, did a much better job, I think, in getting the point across, here:
http://www.survivalblog.com/2009/12/antibiotic_use_in_teotwawki_by.html

Using the wrong antibiotic isn't almost good enough, it's worse than doing nothing at all.

Aside from the risks of giving a drug (and all drugs have significant hazards, including aspirin or herbal remedies), you are using up a precious resource that you can't replace...to no purpose.

Also, giving the wrong antibiotic can cause the rest of the bacteria that are normally in your body to become immune to it, if those bacteria share that immunity with other bacteria that you may pick up that can make you sick, you're in deep trouble...now you're sick with something else, and the antibiotics are gone. Or, it may just cause severe diarrhea because it knocks out the normal bacteria in the gut (the ones that are necessary for health), and on top of a bad infection you now have diarrhea - getting dehydrated, and screwing up your electrolytes.

Finally, people take antibiotics far too often. Most infections people get routinely are viral, not bacterial and there is no antibiotic that works on viruses. There are a few antiviral drugs, that don't work very well at all: We can cure bacterial infections (most of them, anyway) but we can't cure viral infections (the most we can hope for is keeping you alive long enough that your body cures itself - or we can prevent a few viral infections, through immunizations (some of which work against bacterial infections too).
When I get a patient with an earache or sore throat, for example, I usually will write a prescription for an antibiotic, but tell them to wait until they've been sick for 3 days to fill it....usually they have a viral infection and it will clear itself: If it lasts 3 more days it's probably bacterial and the drugs will help.

There are times when I think antibiotics are good: If someone gets a penetrating wound to the chest or abdomen I'll load them up before surgery, if they have an open fracture I'll give them a different antibiotic regimen, if they have certain problems (no spleen) I'll be more willing to give them antibiotics than others. But they are a powerful tool, and need to be treated like that - I tell my students that antibiotics are like a running chainsaw and deserve that much respect.