Alot of DP members are successful with yahoo answers . but they start to ban users who are having more than one account .
its good coz' you can noticed that most question that you can think and search it on google, yahoo answers are there...
Yahoo answers can give you a good boost. But just like all other search engines, abuses are prohibited. Rule of thumb??? Don't abuse it.
Create only one account and answer all type of answer.. if you promote your site in one of the answer, then also reply to few other question without promotion of your site which make it natural.. but if you start promoting your site in every answers then you might be banned.
yes its good to have only one account because you can concentrate to get the account credibility higher by making post only from single account instead of multiple accounts.. reputation point is surely be gained due to this.