Dedicated IP's are useful if you want to use microsoft frontpage to publish your website before your domain resolves to our servers. This is also necessary if you want to be able to view your site via http://IP instead of http://IP/~username/ before your domain resolves.
Once your domain resolves to our servers, there is no difference between dedicated IP and non-dedicated IP accounts.
Also if you like to use private SSL, you are required to have one dedicated IP to install private SSL.
If you need a dedicated IP, please open a ticket at sales department.