I see no persuasive reason to not start Smith at this point. He has versatility and contributes in all aspects of the game - something I think some people underestimate. Sibert might average more points and make more 3s, but as far as all-around production, Smith has that aspect possibly over Sibert. Thompson is backing up Buford at this point, so the roles there are somewhat clearly defined.
Now, perhaps in the future if Ross is on the team than maybe Thomas' starting spot could be in jeopardy, because I think the 4 is where Ross would fit in best on this team & if he's as good as advertised, then he could be a better option than Thomas.
If/when Ross is on the team, we could see a fairly well-defined and simplified rotation with 1 starters and 1 backup playing at each position - if Craft, Smith, Buford, Thomas, Sully is the starting 5, then the backup group could be Scott, Sibert, Thompson, Ross and Ravenel.