When you use a registrar's redirection service, the registrar's server returns a 302 - Moved Temporarily
HTTP response code.
Search engines like Google and Yahoo interpret this code literally - resulting in a multiple instances of your domains (i.e. example.com, example2.com, example3.com) in their ranking indices.
The effect of using a domain registrar's redirection service is negative - because there are multiple instances of your domain's content in the index, no one instance receives the ranking which should be assigned to it.
This is a very well-documented behavior - I suggest that you consider the following discussions of redirect behavior as it concerns search engines before you consider using a domain registrar's redirection service: