Because Hammer of Sol should not benefit from Helm of Inmost Light. It is class specific perk to free up another good synergy with that class, or provide an equal, universal benefit to each. Obsidian Mind should not refill Stormtrance or Radiance. If you want to use a universal mobility exotic, use Radiant Dance Machines or Bones of Eao. You already have plenty of choices. We don't need to buff or nerf mobility of all things. Finally, Shadestep grants invincibility and deactivates homing attacks. Twilight Garrison does not. They are not equal as they do not have the same properties. Thusly, the same argument for one does not necessarily apply for another. "Urine shouldn't be swallowed just because Lemonade is."