Much like Active Directory forests, you should only add a second Shared Services Provider (SSP) when absolutely necessary. Adding another SSP greatly increases the complexity of your overall design. For example, adding a second SSP infers the creation of another user profile database, Search services, My Sites host, and much more. However, there are a few reasons to create multiple SSPs:
Legal, regulatory, or government regulation
Geographic dispersal: you cannot use intra-farm Shared Services over a Wide Area Network (WAN) link with a delay greater than 80 ms.
Indexing limitations, such as geographic, size, or WAN latency
Extranet or Internet needs, such as an Extranet portal or Internet publishing site ...