Resending this because it seems to have gotten lost along the way (at  
least from what I could tell):
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The subject states my specific objection to the current behavior.

Yes, for many languages, it takes the same number of keystrokes to  
insert a comment character; however, there are languages/modes where  
this is *not* true (for example, C89 or anything based on XML).  
Additionally, it makes sense to have comment-dwim handle all cases of  
managing commented material and comments.

The proposed diff adds the following additional behavior:

If the point is at `line-beginning-position' (and the region is  
inactive), call `comment-region'
  on the whole line (unless the line consists of comments, in which
  case it calls `uncomment-region').

The patch is attached below: