Some of the "no" points suggest that there will be a high concentration of green house gasses. This is not always the case, the city can always reduce their intake of those gasses and contain a fresh atmosphere. As for the 2nd no point, violence can be easily taken care of by a police force. In terms of diseases, they can be spread in just about any region. (Depending on the strain.)
Yes, urban density is actually beneficial to society as a whole. In places where density of population is high, there is a greater need for community resources such as public transit, public libraries, and other beneficial mechanisms of society. Alternatively, studies have shown that when urban density is low, the usage of energy is actually high! Urban density is not simply good for society, but it is also good for the planet.
As many commentators have pointed out, in the globalized world connected by free trade agreements, urban centers have become more important and more densely populated than ever. Some critics have attacked the place of the city in the new global economy, pointing out how urban centers consume a disproportionate amount of foodstuffs and raw materials without producing any of them. To these critics it should be pointed out that the trade-off is the potent concentration of human resources that urban centers provided, and the consequent increase in innovation and productivity.
No, urban density is not a good thing. First of all, packing people together too densely raises stress levels and increases likelihood of conflict. Crime levels are therefore likely to be higher in such an area. Second, the more crowded the environment, the more problematical cleanliness and waste disposal becomes. Finally, safe and orderly public response in a densely populated area is more difficult in the event of disease epidemic or disaster.
I think it's ok, but not necessarily a good thing. Having dense neighborhoods contributes to disease spreading, violence, and things like that. Humans definitely like to have a sense of their own space, peace and quiet when they want it. Too much density is not a good thing in my view.