Ignoring a username, or a thread, or an entire sub-forum isn't about making them disappear (which implies some action against the ignored) but is just a simple filtering of information to see that which you wish to see, whether to decrease personal frustrations and bugbears, to increase volume (ratio) of information relevant to you, or to "follow" topics of most interest. Within the limitations of the forum software, Alex has tried to enable as many filtering options as he can to allow personal customization of experience for maximum benefit & pleasure and least frustration. Ignoring something, filtering it out, is a passive means of doing something, and we all do it regularly (even subscribing to MobileRead is filtering your attention towards a preference).
Everybody is different, and has different means and requirements and assets and time to put towards the filtering of information. Things like ignore-filters are methods of increasing that. For instance we know many people find the fact that the Politics & Religion forum is "sandboxed" (invisible unless you join its Group) to be a fair means of dealing with those typically controversial subjects while not outright preventing them - it works (or seems to) for the maximum number of people's satisfaction, those interested in those topics, and those definitely not.
Ideally the extension-and-script solution David Marseilles talks about above (thanks for that, David - it all helps) could be implemented at a forum software level, but at this stage it isn't (and I don't know whether it is possible, at either a technical or resource allocation level, or...). In the same way that actively telling people you are ignoring them can ironically turn a passive solution for yourself into an active attempt to personally attack them, so can telling people that make use of the filtering abilities of Ignore functions for users, sub-forums and threads that they lack some essential ability (when in fact they are making use of solutions that try to personalise themselves to the inately and delightfully individual personality, character, emotions, interests & circumstance of each user).
The Ignore function could always be better (if only there was a way to filter out all copyright discussions...
), and suggestions on improving it, whether through MR or via external means as David suggests, are always welcome. But the motivations behind the presence and use of the tools are too broad and individual to make sweeping statements about its necessity.
In the end Ignore would never at all be necessary except for information-filtering, all Guidelines would always be followed (particularly the one about politeness), and we'd all get along in comfortable agreement like a coterie of Care Bears. But that's not the way things are, but only how we can try to be. So in the ragged gap there are lubricating features. Encouragement in their use is, I think, a constructive step.
Cheers,
Marc