C.E. Evans wrote:
I think being a starship captain carries an enormous responsibility--at any given time, entire worlds and millions (if not sometimes billions) of lives could depend on the decisions he or she makes. So regardless of the size of the vessel, the billet for the commander of a frontline Starfleet vessel could indeed warrant a captain. The exception may be with small support craft or small starships assigned to a starbase (in the latter scenario, the ship would perhaps operate under the authority of the starbase CO, even if a lower-ranked officer was in the center seat).
I personally think this is the most likely scenario, both logically (why would you place the lives of a planet and its billion of inhabitants in the hands of a guy two years removed from the academy) and from what we have seen on screen. And by that I mean this, to the best of my knowledge absent wartime, or the captain being dead or otherwise out of the picture have we ever seen an onscreen ship commanded by someone of a lower rank than captain? (Please enlighten me if I am wrong)
In today's navies, to the best of my knowledge (and please correct me if I am wrong), no major combatant is commanded on a permanent basis by anyone of a lower rank than commander. And the most important ships, missile subs and carriers have full captains on board. A lieutenant can probably command a patrol ship very easily--especially if his XO is chief who's career started when the LT was in diapers--but that is a whole different ball game than trusting a mission to hunt down and sink another boat or ship, or a cruise missile or air strike to him.