We need more voices on our side. We have a fair idea how Trump would have handled the talking points. Not having Trump on the panel lets Vivek shine. I honestly don't see Trump adding to this debate. He and Vivek would dilute each other's performance.
IMO the debate isn't necessary because of Trump's lead and because we have knowledge what each candidate thinks. The purpose of the debate IMO is to have a backup candidate if Trump literally is unable to run.