The Point of Ventless:
Is to distribute kitchen grease and fumes more evenly throughout the kitchen, instead of allowing them to concentrate solely on the wall and cabinets near the range.
Seriously, they're intended for installations where venting is impossible for some reason, but builders often install them to save themselves the cost of a vent run. They depend on very good maintenance to do any good at all, and they almost never get it.
If it were within my power to do so, I'd outlaw them. Once they're neglected for a while, they become grease-laden fire traps, and their inability to exhaust moisture (whether maintained or not) causes damage to houses.