Heroic past lives are a waste of time. An epic past life will grant, lets say melee, a toggle for +1 to all skills, and +5 ac at level 30, a fighter past life will grant, +1 damage. and +2 to sunder. A racial past life will give you +1 to a random skill, or +1 to a random stat. So racial, that's a benefit of +1, epic, +26, and 5 ac. And oddly, it will take you 3 times as much effort to go from level 14 to 15 as it takes to go from level 29 to level 30. Need I add, a heroic incarnation requires 20 levels verses 10. It's just math.
The game was built with 2 different systems, heroic, and epic. They realized the game was to grindy when they developed epics, and so they made a much improved system. They never adjusted heroic levels. Until they do, you should play epic or iconic lives.
(if you want heroic class past lives, you get them in iconics. So, shifter, thats +1 to hit and damage, +1 will save and a heroic past life, with half the time invested. Oddly, it will take as much time to go from 15-20 as it does to go from 20-30)
While I am at it, I should mention the money investment. Epic and Iconic reincarnations are basically free. For a heroic incarnation, you have to farm tokens, or you have to pay for it. To farm tokens, you have to play a few select dungeons, which generally are low xp for the time invested.
The basic problem is that heroic incarnations have a punitive xp penalty. Epic levels do not. So there is an easy solution, remove the xp penalty. (This is why I get trolled, everyone who grinded through the heroic completionist loses their mind. They want everyone else to walk a mile uphill in the snow)
I'm sure, people will say I am arguing toggles verses feats, let me counter. If you are a new character, and you don't have the toggles, the reward is exponentially more valuable. If you have the toggles, well 5 ac is still better than most heroic rewards, definitely better than racial. To go further, some toggles are better at low levels, other at higher. For instance I use the +9 MR till 26, then my MR is maxxed, and I use the spell crit
Ultimately it comes down to diminishing returns. After 1 or 2 epic completionists, Racial incarnations are more appealing, not because of the rewards though. They are appealing for reaper levels. When you can comfortably solo reapers, the first time XP bonus is a nice benefit. That makes the time investment more worthwhile. Its multitasking, but realize, at this moment, its not about the incarnations (And really, it is much easier to solo reapers in epic levels) however, doing both is appealing. (btw, this is why 5 ac is appealing. Don't listen to these min maxxers who say otherwise. Defenses are multilayered, you will never be 100% invulnerable)
To conclude, there are some valuable rewards for heroic incarnations, but, for the time invested, you are far better off to focus on epic and iconic levels.