The conventional wisdom is that you should punch up, not down. In other words, if you're behind the market leader, it's OK to throw a punch at it in your ads, because your product isn't as widely known or understood as theirs is. But if you are the market leader, you don't throw punches at competitors, because you're already winning and doing so just reminds the viewer/reader/whatever that other alternatives exist.
This mitigates the "defenseless kid" syndrome a bit, because if you only punch up you're by definition only taking on products that are stronger in the marketplace than yours is.
You can see this in action by comparing Apple's ads. They had no problem running the "I'm a Mac" ads when their product (the Mac) was far behind the market leader (Windows PCs). But iPad ads don't do product comparisons, because the iPad owns the tablet market so there's no upside to reminding people that "tablet" does not necessarily equal "iPad."
Coke vs. Pepsi is a classic example. In the Cola Wars, Pepsi, from time to time, punches up against Coke far, far more than Coke ever attacks Pepsi. Coke certainly increased their advertising in response to Pepsi in the height of the cola wars but they never attacked Pepsi outright.
Comparison advertising is also a "marketing smell" (by analogy to "code smell"). If you're doing comparison advertising, you're probably in a Cola War. Cola wars are a negative-sum game where you only have to increase advertising if the competition does--sort of a prisoners dilemma. It's preferable to avoid getting into those.
This mitigates the "defenseless kid" syndrome a bit, because if you only punch up you're by definition only taking on products that are stronger in the marketplace than yours is.
You can see this in action by comparing Apple's ads. They had no problem running the "I'm a Mac" ads when their product (the Mac) was far behind the market leader (Windows PCs). But iPad ads don't do product comparisons, because the iPad owns the tablet market so there's no upside to reminding people that "tablet" does not necessarily equal "iPad."