should zoos be banned?

Quite simply no. While there are, naturally, a number of very bad animal collections in the world that doesn't make zoos bad (any more than the fact that they are corrupt cops means we should get rid of the police!). yes, many should be closed and many need improvements, and there are some species which chouldn't be kept. However, zoos are good in general:

Zoos provide a huge resource for educating and entertaining the public. Most people will never see most animal speices in the wild, bu they can see them and learn about them in zoos. They also encourage the public to invest in research, conservation and the like, all invaluabe things.

Zoos provide a huge resouce for scientific research. We can observe lots of behaviours of animals in zoos and take things like blood and DNA samples. We can do simple and harmless research like seeing which trees an animal might prefer to feed on, or which diet they prefer which can be of huge benefit to saving species in the wild. Those that die can provide specimens for museums and reseach.

Fianlly zoos do provide a protected reserve. Species have gone extinct in the wild but been protected and saved in zoos and later returned. Animals that 100 years ago would never have survived have been lost (or near lost) and are now alive and well becuase of zoos. That is really quite incredible and cannot be achieved any other way.

No.  Zoos are awesome.

They should be regulated though, and in cases better than they are now. Ironically though it a great many countries, zoos are regualted by the governement and self-regulated by associations and checked by independents, and yet private colelctions of animals or even simply pets get little or no protection at all. There are indeed bad zoos out there, but people who decry zoos as fundamentally bad for animals should look at intensive farming practices and general cases of animal cruelty.

They should be regulated though...

There obviously has to be some level of regulation, if only to make sure the polar bears don't break free and decimate the population of the local primary school (I'm being a bit facetious, of course, but there are some real safety concerns). I would lean towards a relatively light touch, though. Zoos are awesome, as Mike said, and burdening them with excessive costs and red tape is going to limit their ability to do all the positive things you mentioned in your first post. Also, poorly run zoos are not going to be as popular with the public anyway, so the people in charge have a built-in incentive to maintain high standards.

Very true, especially in the modern world where so many visitors value conservation and animal welfare. They *want* to know that they are patrons to well-run and useful institutions.

Also, poorly run zoos are not going to be as popular with the public anyway, so the people in charge have a built-in incentive to maintain high standards.

I'm not so sure that poorly run zoos are necessarily that much less popular - it depends on their focus.

Zoos have to maintain a careful balance between the welfare of the animals and access to the public, which is tricky because the needs of the two often don't overlap very much. A zoo that's well run for the animals may put off the public because they don't get to see the animals much when they visit. Conversely, a zoo that's well run for the public may be poorly run from the perspective of animal welfare.

The problem is that for a commercial enterprise the paying public are where the funding comes from, which may mean that public facing side of the zoo is well run, but the animal welfare may be neglected. This is where regulation comes in, providing a greater incentive for animal welfare side of operarions.

Of course, Dave makes a good point when he says that the public now has an increased interest in welfare, although I'm not sure how widespread that change of perspective has been around the world. Regulation has its place!

Another thing to note is a lot of zoos are also involved in conservation of animals in their natural habitat as well as the captive breeding programmes that they run that may also result in the release of animals back into the wild where this is appropriate.   

Many zoos do a lot of good, however, I do tend to come down quite heavily on the regulation side - as Paolo said, just because a zoo is popular with people this unfortunately does not mean that it is necessarily good for animal welfare.  Often members of the public may (understandably) not be aware what constitutes good welfare for an animal (it's not always intuitive).  This is a case where good regulation is vital.

On a realted note, zoos provide training for said conservation work. Handling animals in a captive environment safely (for people and the animals) is the best way to learn to deal with wild animals that need to be captured and handled (like reintorductions, rescues etc.).