In this kind of situation, where there is one far-reaching ability that may have heavy consequences on basically the whole campaign, my main goal has always been consistency with the original thread. You take a similar monster and you compare the current monster with it. Case in point, incorporeal undead. The spectre got a LA+1 in the main thread, for a total of ECL 8. Now, offensively, Charisma Drain and 40ft flying is much worse than 2 negative levels on your incorporeal touch attack and 80ft, but comparing that with the defensive abilities of +8 Dex, -6 Int, +6 Cha, +2 Turn Resistance, no sunlight powerlessness, death's grace and freaking Despair, I can't put the deathshrieker below the spectre. Not that much higher, but definitely stronger. 9 RHD is a minimum in my opinion, but putting the deathshrieker higher would be a stretch, since it is really passive without at least one or two class levels. So there you have it.
Of course, the standards for Level Adjustment have slightly changed during the years, and maybe the Spectre would have gotten LA+0 nowadays, but that's the best scale I've got for some of these monsters. I've never actually played an incorporeal creature, you know (the closest I've done was a Shadowcaster/Master of Shadow), but even with that I know that it is really a boon, especially with dedicated stats like those of the deathshrieker.