Should I develop a custom ASP.NET membership and role provider or not?

Hi,
I need to store membership and role information in a Firebird database and therefore know that I should be implementing a custom membership and role provider if I wish to use ASP.NET membership and role management.
However as each role for example has to be associated with a particular organisation, and the CreateRole method that I will implement in the custom provider takes only a roleName as a parameter, does this mean that I need to be implementing an overload for the CreateRole method that takes organisationName as an additional parameter?  Each user will also be associated with a number of different database tables.
Or should I just ignore ASP.NET membership and role management all together and implement authentication and authorization myself?
The client is being implemented in Silverlight.
Thanks.
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