The thing is that Bioware has actually balanced the quests really well, when it comes to the amount of XP you gain per planet, by doing all the quests and it usually amounts to the exact number of levels that the particular planet has been marked for. However, there is a lot of extra content, like space mission, pvp, flashpoint, and heroic missions and it can be pretty easy to out level the planets if you do the extra content. This is a good thing though, because not everyone has the luxury to do all that extra content. Me for example, not really into pvp so I don't do warzones and, even on active servers, it can be tough to find groups for heroic missions. Flashpoints also are pretty difficult to get to because they are at the fleet and nobody wants to go and just sit there, doing nothing and wait if you can get a group.