I think using football to compare to soccer doesn't work for the argument. In the NFL, or football in general, the defense can score points that can win teams games, or can put the offense in a position where they'd have to be completely incompetent not to score by forcing turnovers. Additionally, even the worst offenses in the league tend to score once or twice in a game. In soccer, the defense can't score no matter how good it is. Tactically, I also think it's easier to keep opponents from scoring with a "bad" defense than it is to score with a "bad" offense, as scoring is difficult to begin with.
For arguments sake, lets say a team focused completely on defense first in MLS with DPs. There's a good chance they'd keep a lot of clean sheets, but if they can't score with a mediocre attack (which is probable) it results in a ton of draws. While keeping a clean sheet and hoping to find a single goal to win might be a great strategy in the playoffs, or another knockout competition like smaller teams try in the FA Cup, for the regular season, it might not earn enough points to even qualify for the playoffs, like Chicago last year.
For us, though, I think we need to focus on the roster we already have. Conventional wisdom we have a pretty strong attack, headlined by Villa, with a bunch of other good, but not great players. Our midfield is considered a strength, and many would say is top half of the league at worst. The defense, though, is questionable, and without Meara many have doubts about the keeper position. So the question is, does making a substantial improvement to our defense at one position, be it DDR or someone like him at CDM, DeMichelis at CB, or a quality GK, give us a better chance of winning matches than making a smaller upgrade at an attacking position, such as a forward to pair with Villa or someone else in the midfield.
I'd put the order of need for us like this assuming our 3rd DP is a well above average MLS player.
CDM
ST
CB
GK
LB/RB
MID
If a well above average CDM is not available on the market, we move to the forward position, if we can't find a well above average forward, we move on to CB and so on. Basically, instead of compromising the quality of the player to make him play an attacking position, I think we need to get a very good player that fits our need most. If DDR, a player like him, or a class forward isn't available, then I have no problem signing DeMichelis unless our needs change. In fact, I'd rather sign DeMichelis than make a minor upgrade at forward or in the midfield by signing a less than well above average player.