While it's true that no dialect is inherently superior to another, the level of prestige attached to a particular dialect by society can vary wildly for no obvious reason. Whether we realize it or not, this external value is the only reason to prefer one dialect over another. It is a glaring and malignant mistake to assume that a speaker of a given dialect is necessarily less intelligent or less friendly than a speaker of another. If a person speaks in roughly the same manner as their parents, their siblings, and their friends, this is perfectly natural and says much more about the individual's geographic location than their mental capacity or personality.
Linguistically, it makes little difference whether you pronounce the 'z' in the Spanish word paz like the 'ss' in the English pass or the 'th' in the English path. The only objective measures for a difference in pronunciation is the markedness of the relevant speech sounds and their potential for distinguishing would-be homophones. If the former criterion is used, the Latin American 'ss' pronunciation wins out because of its ease of pronunciation. If the latter, the Castilian 'th' is superior because it makes possible a distinction between words like casa and caza. A similar situation exists in American English with the vowel sound occurring in words like paw and caught. In the Eastern US, people tend to distinguish these words from words like Pa and cot, but in the West, such words are homophonous. Speakers of most eastern dialects have the advantage of being able to clearly distinguish such words by pronunciation alone, but at the cost of one more distinction to make in an already crowded vowel system. As you can see, when truly objective measures are used to compare two dialects, no clear victor emerges.
Having pointed out that dialectical beauty is in the ears of the beholder, I must also stress that what dialect a person speaks is, to some degree, a choice. I grew up in a small town in east Texas. Both of my parents have a fairly noticeable Texan accent, and I was exposed to a lot of regional vocabulary items such as skillet for frying pan, commode for toilet, and coke for soda. I was easily identifiable as a Texan until somewhere around the onset of adolescence, when I apparently became more self-conscious. I didn't want to sound like a Texan, just an American. Since then, I've consciously and deliberately made several subtle adjustments to my pronunciation and vocabulary so that people wouldn't mistakenly perceive me as a "hick" or a "redneck" as soon as I opened my mouth. While I can't assume that people who haven't chosen to make similar adjustments are less intelligent than myself, I can make certain guesses about their social identity. It is for this reason alone that not all linguistic profiling is entirely unfounded.