SAML 2.0 pseudonyms

["Moniava, G." <g.moniava@student.tue.nl>]

Hi,
 
I have a question about pseudonyms in SAML 2.0. Particularly, as I understood
SAML 2.0 doesnt state ways how the pseudonyms should be generated, rather
it just offers means of transporting them to requester parties. If that is true, than
why could not we use, simple <uid> fields for transporting these pseudonyms.
Are there any benefints for having seperate, support for pseudonyms in SAML 2.0 ?
 
Thanks !