The reason I canceled accounts over one negative feedback is they were usually low feedback accounts at that point. -1 when you only have 100-500 feedback points is enough to lower auction prices. I list everything I sell at $.99, even thousand dollar camera setups and stuff like that. One negative could affect the final price by 5-10%.
Because of ebays new policies its fairly easy to set up a new account and become a 'power seller'.
100% feedback is not needed for a personal account, but for my business its crucial. Almost every item I sell with my 100% powerseller account is sold at a higher price than the ebay average for that item, sometimes significantly higher. This is partly a result of the $.99 listings, that leads to a lot of early on bidding and higher search results (becuase of ebays new fluffied up search system), and in turn generates more "watchers" and more interest, leading to last minute bidding wars. Its ridiculous, sometimes people will get in bidding frenzy at the end of auctions and pay 100% of full retail, or more! This is the reason my few customers pay me a good commission to sell $10,000's worth of their gear rather than doing it themselves.