Akismet can be used free of charge, but only for websites that are not for profit, or charities. You won't be getting any benefit from Akismet unless you get a security key and add it to the akismet configuration page on your site.
I don't think that Akismet will stop spammers from registering on your buddypress-enabled site.
I would also like a resolution to the wds_bp_registration_options_bp_before_member_header warning problem please.