The problem is, I don't think we have any real strengths to deal from. We just aren't good enough yet to have that surplus. And really good defensemen are seldom available, let alone cheap. Therefore, by using Brisebois or Streit (or even Bouillon when he's at his absolute best) as an internal patch on the wished-for #4 guy, we're at least not opening up the other gaping holes in our lineup that trading for such a player would create.
And also, I think once Markov and Komisarek really get into their usual groove, it will make the D seem even stronger.
We're just not there yet... no need to try to "fine-tune" something that isn't even fully assembled yet.