Feature request 'answers' aren't really answers
While they remain called 'answers' the answers on feature requests aren't really answers. They are reasoned arguments for or against the proposal or suggested modifications to the initial proposal. As such the aim of a feature request is not to acquire answers; in fact a feature request that is obviously a good thing and is well argued in the initial proposal may never get an answers at all (although once implemented it may get a stub answers indicating which build it was implemented in).
It helps the feature request to be closed as a duplicate
Given that the stated aim of a feature request is for the feature to be implemented the greatest thing that can help that aim is to show wide community support for a proposal. This means collecting all the votes for a proposal in a single place; this means voters can be directed to the master feature request - a single feature request with +200 is far more powerful a statement that 20 identical feature requests with +10 each.
The ultimate aim of a feature request is to be implemented not answered
Given that the aim of a feature request isn't to be answered but to be implemented and acquire a status-completed tag the requirement for an answered question to be marked as duplicate is removed.
Answers to bug reports don't really make sense
Bug reports need answers even less than feature requests do and have answers even more rarely. The only exception to this is that the build in which the bug is fixed is often posted as an answer; however this is only done after the bug is fixed. Sometimes detailed bug reproduction steps are posted as answers but this is rare and only occurs when the original bug report has failed to include them.
It helps the bug report to be closed as a duplicate
When a bug report is closed as a duplicate of a single master bug report it allows all the votes on that bug to be collected in a single place; allowing it to be quickly observed how important this bug is to the community. It also allows all the relevant information on the bug to be collected in a single place (for example systems effected) rather than being spread around several unconnected reports.
The ultimate aim of a bug report is to be fixed not answered
Again since the aim of a bug report isn't to be answered but to be fixed and acquire a status-completed tag the requirement for a bug report to be marked as duplicate is removed.