Need? No, it's just another option. Downdrafts reportedly don't work as well on gas ranges due to the sideways drafting but work just fine on induction and/or electric cooktops. So it's whatever fits your kitchen layout.

Just treat it like a regular cooktop when picking out ventilation. I wouldn't do a downdraft unless I had to.

downdraft far inferior compared to typical hood, for gas, induction, or any other heat source. Ever see a fireplace with a chimney flue located below the logs?

Absolutely no reason to switch.

I saw this advertisement on my gmail so looked at it. I wish I knew if this is a good hood. Have any of you heard of this brand?

Here is a link that might be useful: ProLine Range Hoods