The problem as I see it, is that believing oneself to be "relatively uninformed" does not make a person relatively uninformed (it may in fact lead them to actively acquire more information - i.e why Severs believes he is below average). Conversely, believing oneself to be well informed does not make one well informed, and may alas lead a person to stop seeking relevant information.
Indeed it would not surprise me if the people most affected by "simpleminded sloganeering" tend to believe they are well informed.
The problem with "people should" in a political context is that it can quickly become "people must" backed with the force of government, and any implementation of: "Informed voting > not voting > uninformed voting" requires disenfranchisement.
But, if someone has enough self-knowledge to recognize that they are relatively uninformed, on a single issue or the ballot as a whole, they should abstain from voting on those portions of the ballot. By doing so, they delegate the decision to more-informed people.
The option to vote if they are later agitated or informed remains important -- but it doesn't have to exercised every time to serve its purpose of defending their interests.
People moved to vote at the last minute by a speech about their 'obligation', and deciding based on a few fleeting impressions from the last wave of rhetoric, make the process more random and superficial. They are the people most affected by negative ads, or simpleminded sloganeering.