Just because people do things, or do them regularly, does not mean they are okay.
Any animal that is intelligent and self-aware has the capacity to suffer. By all means, we should not impose suffering for no purpose where it can be avoided.
I posted in another thread about why scientifically, cavies are a terribly inefficient food source relative to larger vegetarian animals. There is no way to justify this suffering in terms of eating Cavies.
As far as eating other animals, there is a concept best described as the 'circle of nature', ie. out of the 'lion king' movie by disney if you want a popular culture reference. Like we discussed in the other thread, a cow or other large vegetarian (goats or other animals where cows are not viable) does serve a purpose in efficient food production and use of land. People can't eat grass, but cows can eat the grass, and then people can eat the cows, etc. Too much agriculture -- overfarming -- can ruin the natural environment of an area.
Too bad that's the old type of cattle raising, though, modern cattle farms (vs. free range cattle of the old west) are just as damaging to the environment, if not moreso.