I have been leveling as Defensive and have noticed no problems whatsoever. I spec for defense, gear for DPS, and overall it has evened itself out beautifully. I have had no problems tanking any of the flashpoints I have tried, nor have I had difficulty through the story missions. I run with Kira all of the time, rarely use my defensive cooldowns on anything other than big fights, rotate my limited crowd control on group fights, and usually can make it 5 or so encounters before I feel the need to rest and heal.
Honestly, the only issue I have ever had with staying alive and healthy is the fact that I frequently try and push the level/rank of mob I go after.