I have never noticed a need in Little Rock to use them as we have fairly soft water.  But my parents live in deep South East Arkansas and have fairly hard water.  They have a Kenmore Ultrawash with the "Smart Wash" sensor.  That is the only cycle they use.  We do notice buildup occasionally and the Finish cleaner always removes it.  The dishwasher is over 6 years old and still runs like a new one.  It even cleans better than the Ultrawash it replaced tho it does take longer, I attribute both to the sensor.  But, I digress, to me the cleaner works better than just plain vinegar and seeing how they only use it about 2-3 times a year it seems very cost effective.  

I would like to say that it depends on where you live and the conditions of the water, etc.  For my parents it is necessary, no amount of detergent or hot water can prevent the lime buildup that they have down there.  For their needs a cleaner is necessary and vinegar wont cut it.  For my water conditions it is not necessary.