What does Upgrading Huntress Invisibility Do?

Most skills in Dungeon Defenders have a logical 'upgradeable' feature. Damage, range, speed, etc are all numbers that can increase (or be decreased) logically. But the Huntress has her Invisibility skill, which has no intuitively scalable difference. It's on/off--visible or invisible.

What benefit does upgrading this skill give a Huntress?

Solution 1:

According to the wiki:

Putting points into this skill will decrease the amount of mana it takes to keep the skill on.

If not leveled, you'll need 15 activation and 4 mana per second which will slowly increase the longer it is used.