What are the advantages of using one of the Sparkpost extensions for Civimail?
With sending emails there is always the sending, and the return path. If you configure the Sparkpost SMTP server settings in CiviCRM you've resolved the sending, but not the return path. So you will not get the bounces and might end-up in trouble with Sparkpost or on blacklist because of repeated mailings to bouncing email addresses. You will still get click-through and unsubscribe as Sparkpost will forward to CiviCRM all links that users click, even if it intercepts these.