A weighted index is like a family with eight children who only considers the males important. They have 2 unusually healthy boys and 6 average girls. If the boys are healthy and three girls are sick, when grandma calls she is told, "We're all fine."
Thus it can happen, and commonly does, that most of the 30 stocks on the DOW are down for the day or flat, IBM and two or three other weighted leaders are up, so the DOW closes up several points despite 27 of its 30 stocks being down or unchanged.
Since most of the news media reports the stock market by reciting how the DOW finished, and perhaps adds the Nasdaq close, the report conveys false information about how the market did that day.
When these indices came into being stock investing was done by relatively few. Attracting wealthy individuals away from their local bank trust department was job 1 for Wall St. The result of weighting the strongest companies is to make the market look stronger than it is, thereby attracting investors.
In an era when too many rather than too few invest, when accurate information is essential, weighting has outlived its justification.
Today such misleading indicators serve primarily the bucket shop telemarketers preying on the elderly, and the newcomers to stock sales assigned to sit at tables in Florida malls and grab the arm of passers-by, the kind of salesmen we are told "don't represent the industry."
On the contrary. Because of the wide public exposure of such sales people, and because the industry aids them with falsely weighted indices as sales tools, the entire industry is tarred with their brush. They DO represent the industry.