You hit the nail on the head right there. Most of the XP from doing a FP comes from the reward for the mission for completing the FP itself, bonus missions and the daily terminal mission. Killing anything that can be avoided that isn't needed for the bonus is just a waste of time, you would gain much more by going back to whatever planet you where on and use the time saved to kill mobs while also doing normal missions.
I disagree with the above statement. If you've spent any credits on the Bonus XP for FP's, while lvling if you kill the majority of the mobs in FP your going to get more XP than running around a planet killing mobs. If you q at a proper lvl you can bang out about 1/3-1/2 of a lvl with just killing mobs. If you do bonuses and have the daily quest and the quest to complete said FP you can gain almost 1 whole lvl in roughly 30-40 mins. I'm not talking going in and killing every single mob in the instance, but if you kill the ones in your path it's well worth the time spent.
Actually I took about a month+ break and just started playing again sunday. Last night I qed for a FP with my 33 Mara. It put me in Cademimu which I over lvl. I wasn't pleased but hey I wanted some PVE. While going down the big elevator the healer jumped and killed himself then rezzed. He said wait im the healer, the tank responded by saying it's ok were skipping all this, we'll be fine pulling the next mob. I then said " I don't care because I over-level this place and im not getting much xp anyways, but what is the reason behind skipping all these mobs while lvling, you need xp. Got a classic response "Just F'n leave". The tank tried to get me to leave the entire FP talking trash telling me i'm a typical mara and I don't understand anyting, im just a noob. Yet I was the only one killing weak to strong.Actually getting yelled at for not attacking his target.He wsn't using guard and if I remember he got his AOE taunt at 29 for a PT which I never saw used once he was lvl 30. I just laughed him of tho, I have all 3 tanks and have cleared all content(except new ilum) on them except on the juggy he's on a different server.