I suppose this whole discussion brings us back to whether or not there are "line officers" in the trek universe. For instance, in "Disaster" (season 5, episode 5), Troi was controversially given command of the bridge based solely upon her rank (over Ensign Ro, whom one would assume would be a line officer in a regular navy).
Starfleet clearly has "command track" and "non-command track" officers, but, other than specifically designated first and second officers, there's no evidence of having line officers in the traditional sense. Lacking line officer rules, LaForge and even Doctor Crusher (who eventually became a captain in the "All Good Things" timeline) may automatically be considered by Starfleet as experienced enough to become executive officers or even to command ships (under the right circumstances) based solely upon their ranks. At the very least, it's clear that the current trek editors feel that way (both given what happened in IFM and that Riker offered Geordi the XO position before taking command of the Titan).