If you have a dormant card, it robs points from your FICO. Depending on how old the account is, how long since it was used, how much the credit line is, and how long was it actiivlly used for. It make take as few as 1 point per card, to as much as 25 points per card. The amount of points that it takes varies from card to card, issuer to issuer, and on each credit model. For example, if you have a Curcit City that is issued by Citi Cards, a Office Depot card, issued by Citi Cards, and a Staples card also issued by Citi Cards. The Office Depot card and Stamples cards are considered dormant, but you always use your Curcit City card. The Experian model my consider them all active because they are all issued by the same comapny and viewed colectivly. The Equifax modle may take points on the two cards considered dormant and not on the other because they are all viewed seperatly. Where as the Trans Union Modle may consider them all inactive because the same issuer did all three cards and is reporting a dormant card. I know this is all very confusing and, at least for me, is frustrating. The scoring modle is always consistant for each company, but they don't have to ever release any information about it to help consumers improve thier credit score.
It pays to do your homework and know which is teh primary and secondary beuru for your state. Also know who is issuing your cards.
As I after thought I guess maybe I should say how I know what I know. I work for Citi Financial. I work primarily as a loan underwriter, but also review credit acrd applications for all of the cards issued by Citi. I work both with consumer and comercial finances.