Ok, without looking up any more information on cockroaches than I already know, I’m going to take a wild stab at this. (Again)
Cockroaches enjoy warm humid environments. I’m taking this from the fact that the zoo has some Brazilian cockroaches on display that are pretty big, and back when the Earth was a primeval swamp, they grew to great sizes unseen today. So, from this I can guess that cockroaches are native to warm humid environments (read: swamps and rainforests). Swamps and rainforest tend to be full of decaying plant debris and the occassional dead animal, and since cockroaches don’t exactly seem well equipped for the hunt with their low splayed bodies, aversion to light, and lack of biting power, I’m going to have to guess that they eat plant matter.
So: Cockroaches fill the role of eating decaying plant matter. Certainly not a unique role, but if some other insect was doing a much better job of it, there wouldn’t be any decaying plant matter left for the roaches and they’d of all died out tens of thousands of years ago. Also, any insect that breeds in the same numbers as cockroaches has to be food for something and someone already mentioned lizards which would be an obvious guess. For that matter, also frogs or toads, birds, small mammals such as shrews, and perhaps even larger mammals once they get hungry enough. Oh, from what I understand, there’s also cockroaches that live in more temperate climates naturally, and you can pretty much replace swamp plants with old decidious leaves and rotting twigs. But since cockroaches were around when the Earth was a swamp, I’m guessing the swamp roaches were around first and the cool weather roaches evolved from the swamp roaches later to fill a niche.
I don’t know if they fill an irreplacable role in the “Circle o’ Life”, except perhaps in their native habitat before they spread to cover the globe. In urban areas, they would fill no role since they lack natural predators in your kitchen (sans the occassional housepet) and even in the streets I doubt much would eat them except rats and perhaps raccoons or opossums depending on how urban the area is. However an urban area doesn’t fit in the “Circle o’ Life” anyway, so the entire thing is skewed.
“I guess it is possible for one person to make a difference, although most of the time they probably shouldn’t.”