Why saving throw for non-duration stuff?

[Stalker0]

To make it a skill check means it becomes possible for SOME monsters and PCs to be significantly resistant while others are almost 100% vulnerable, making terrain more dangerous for certain PC Classes and Monsters than it is for others.

I think JGulick voiced my opinion on it well.

Higher Tier Characters have a lot of options when it comes to this. They have higher defenses, the chance to have the evasion and mettle feat (negating that dark dream problem), the ability to suddenly raise defenses with other powers, the ability to suddenly come back to life even if falling off the cliff would kill them, ability to survive falling off the cliff (a high acrobatics check can cover many d10's worth of damage), etc. All of those revolve around player choices and builds, so a higher level player has some options to negate the cliff problem if they want to take them.