My initial reaction is that I don't think its a good idea to empower non-technical people with the ability to control such an important variable with a click of a button. I suppose they would be held into account by the miners and this would essentially be a node vote which would allow individual nodes to confirm larger blocks easier. The difference may not be significant as an attacker isn't going to be limited by the code and this could be a signal to allow users to vote. This change appears to me to have more political implications than anything.
Quite right. The choice of settings effectively becomes a vote on what you support.
Just as it is now, the miners actually decide what size of blocks they want to create, based on their knowledge of the network and its participants.
As an end user, you could set your limits high enough not to worry about them for the foreseeable future.
And there are plans to make it easier for end users to track the limit growth curves of BIPs they agree with, which makes it easier for them to express their views on the policy.
To prevent them from isolating themselves from the emerging consensus, there will be a warning message issued by the BU client if the user's settings are too restrictive to continue following the longest chain.