Unnecessary for your personal survival? That's true, but it's a very limited perspective to take. Cockroaches are known to affect our species' health adversely (e.g., http://www.ncbi.nlm.nih.gov/pubmed/11240940). If you wish to reduce their and other insects' unnecessary suffering without compromising the interests of humans, a better thing to advocate for is that we use more humane pesticides (http://effective-altruism.com/ea/nx/humane_pesticides_as_the...), not that we refrain from killing them.