They are a waste of time, but they're everywhere.
If your kids don't understand the basics of ads, or advertisers' motives, they'll struggle to respond to ads in a healthy way.
But the idea that "more exposure" to ads makes you somehow more resilient to them is the most ridiculous thing I've ever heard. Does Facebook make people more well-adjusted? Does instagram make people less inclined to be shallow? Does increased time on Twitter make people less vitriolic?
When digital networks have an adverse effect on the human psyche (mainly ad networks and social networks), the solution is very demonstrably not "just watch more and you'll become inoculated against them".