Pesticides allow us to produce more food, which cuts the cost of the food we buy. So pesticides are especially important in places of the world where people can barely afford to eat. But many pesticides are also poisonous to humans, so we have to make sure they don't contaminate our food, water, or air. Pesticides are used to get rid of pests from many places, not just food. For example, wood in buildings is often treated with arsenic or other chemicals to prevent termites. This is relatively safe unless you burn the wood or eat it. (Arsenic is an element, so it's not destroyed by fire.) And preserving the wood with pesticides also saves our natural resources because we don't have to cut down new forests to replace the wood. Another important example is mosquitoes, which carry malaria, West Nile virus, and other diseases. Without pesticides to kill or repel mosquitoes, many people would die from these diseases.

There are safer types of pesticides now, but unfortunately, many dangerous types are also still made, sold, and used. I recommend Rachael Carson's book, _Silent Spring_, to see the harm some pesticides can do. For example, if you pour pesticide into a lake or pond to kill mosquitoes, you kill all the mosquitoes, but you also kill all the fish, frogs, birds, and bats which eat mosquitoes and their larvae. So then, the next year the mosquitoes come back much worse than before.

There's no simple answer to whether pesticides are good or bad, or whether to buy organic food. The more important questions are what type of pesticide was used, and how much? Also, although organic food is better for you, it can cost 2-3 times as much as "non-organic" food. If the cost of an organic apple is so high that you buy potato chips instead, then you're in MUCH more danger from heart disease than you would have been from pesticides.

Pros:
When pesticides are used, there can be less crop loss to bugs. This yields more food, more profit for the farmer, and less waste of land, time, farm equipment use and water for irrigation.

Cons:
Water that contains pesticides runs off of agricultural areas and contaminates groundwater and the ocean affecting the health of humans and the environment. There can also be health concerns to humans directly from the consumption of pesticide-containing produce and especially for the people spraying the crops with pesticides.

In my opinion, the cons outweigh the pros because yes, there are environmentally-friendly pesticides. Organic foods are good for this reason, yes. What I find especially great about organic produce is that often the food was not shipped as far, so by eating locally-grown foods you are cutting down on fossil fuels burned for transportation. The flip side is that small local farms might not be as efficient as larger farms (in terms of water and fertilizer use, especially), so there could potentially be more waste from many smaller farms.

What you are asking is largely political more than scientific. Pesticides are poisons; they do bad things to other animals in the environment as well as potentially to us. Even if you use an insect-specific poison, there are still a lot of beneficial insects that it will wind up killing as well as harmful ones. Still, some are better than others.

Whether pesticide use is beneficial or harmful overall is going to depend upon the type of pesticide used, the frequency with which it is applied, and the scale over which it is delivered.