You get 3 uses of the active rogue past life and each lasts for 1 minute. If I'm adding correctly, at level 30 it adds a total of +24 sneak attack damage for 3 minutes total per rest. That's pretty good but it shouldn't make or break the build nor change the feat's relative value when compared with the others. If you've properly built for the ethereal plane feat, then you'll do fine without it and should still out-dps the others. If you haven't built for it, or want to emphasize some other build potential such as burst dps or CC, then you're better off with another feat.