In a democratic society, people are given wide selection of candidates to government office. But when the same members of a family control the government and they keep being re-elected, democracy is ruined. Although people vote for these political dynasties that is why they win, chances are, they only voted these members of a political dynasty because of vote buying or other reasons. As a result, those not rich candidates who wish only the best of the entire community cannot compete against these rich members of political dynasties. However, I am not saying that all political dynasties are bad, but the majority of them are detrimental to the society because they destroy democracy and they make the government of the people, the government of few rich, oligarchy.
I don't think that evil is naturally inhering within every political dynasty; as long as the dynasty meets the needs of its people in a just and fair fashion, then the dynasty could often be a boon. So families are just great at serving. However, there is always a potential risk that it could slip into hereditary rule, and that shouldn't be allowed. Rule by the people should be enforced.
What you are saying is that democracy is a more efficient political tool than a centralized government such as a monarchy or dynasty. You say it is more effective because it meets the needs of the poor much more. So say there was a democratic vote - 60% vote yes 40% vote no. The minority loses. The minority do not then stand a chance in democratic voting
60% = majority = legitimacy whereas 40% = minority = fail
This article is a perfect example of how the minority lose out when democratic voting makes their future decisions http://www.Washingtontimes.Com/news/2013/jul/24/republicans-use-budget-bills-in-fight-against-obam/?Page=all
No, political dynasty is not evil, because it will work in situations where the philosopher king is good and wise. It is only when successive rules are unwise and cannot be voted out of office that a political dynasty is a problem. A wise philosopher-king is probably the ideal form of government.