If you look at the math inspiration/bloodthrist isnt quite as strong as you would think. Let say during a burst phase all 4 dps are pushing 2k dps, so a total of 8k dps total. BT adds 15% for 15 seconds, so it adds 1200 dps.... for a grand total of 18k more damage, which isnt game breaking imo.
That 18k can easily be the difference in some burst phases. Consider the battlewalker on EC HM Kephess. Having at least one Sentinel/Marauder on this fight is a HUGE advantage if your group is at all close on DPS.