I personally don't believe it's relevant whether or not it's defined as a "right". There are 2 reasons I can think of off the top of my head for treating it as one.
First, I believe we have a moral obligation to see to it that all citizens are provided the right to "life, liberty, and the persuit of happiness" EQUALLY (not depending on how much wealth you possess).
But from a pragmatic point of view, there is one "inherent" flaw in free market capitalism. It has no metric to evaluate the effect price has on demand. The coase theorem describes the problem fairly well, however, it doesn't provide any method of implementing controls. Until capitalism properly addresses the issue, it's pre-determined to fail.
Healthcare is a perfect example of the inflexibility of demand as price increases (and an extreme one). It would very closely represent one end of the spectrum, and perhaps a porsche or Ferrari on the other. All commodities and markets can be measured and controlled using caps on margins, and thereby protect consumers, while still allowing for adequate margins to attract entrepreneurs and investors, and promote competition and efficiency. But the most important benefit of using the metric to regulate business would be cost control of "inflexible demand" type industries. Lets face it, it makes no sense at all to allow economic predators to "redistribute wealth" by preying on consumers of necessities (I use the term necessities loosely.. the metric evaluation establishes a self-correcting accurate measure of how much something is "necessary").
Yikes! I'm rambling again, heh...