Empty inline elements generate empty inline boxes, but these boxes still have margins, padding, borders and a line height, and thus influence these calculations just like elements with content.
Why then doesn’t line-height work for <br> tags? I’m not quite sure… Hmm, there was some discussion a while back (more: 2000, 2001 on www-style (now with search!).
The simple solution is to just move on a do what works I suppose (display:block; addin margin). But it is bothersome.
Hixie has some styling tests in his Evil Tests: Styling HR, BR Elements.
If <br> has been traditionally treated as generated content and not an empty, non-replaced, inline element, maybe in that context (or others), something like <line> does make sense.