I would say that cheap labor isn't a necessary evil, but just necessary. Americans like to make a lot of noise about how China and India have taken all our jobs. But the fact of the matter is that Americans do not value goods made in America, because they're too expensive.
I believe that cheap labor is a by product of poor governmental policies. When the government legislates things like a minimum wage it causes people to demand higher more things with more money, while the companies need to supply this amount at a low price. This causes them to look elsewhere for cheap labor. Thus, the root of the problem is government policy.
I do not believe cheap labor is a necessary evil of today's consumer-based economy. I think cheap labor is an unnecessary evil of today's consumer-based economy. Employers look for cheap labor to maximize their profits but do so at the expense of the quality of life of higher paid employees. Employers can still stay in business without using cheap labor.
I do not think that cheap labor is a necessary evil. I think it is simply an evil, period. People are working their tails off and cannot even afford basic necessities--and this is in wealthier countries like the USA. I myself have been in this position before, and it is not fun.