Sorry. You don't spend $2M-$3M on a guy unless he can pump in 12-15 goals per season (and you'd hope for 20). To say that a DP forward/AM is good on defensive set-pieces is like saying in football that a WR is a good blocker - nice attribute to have but nowhere the cost of admission.
So, in your words, if a team pays DP money to a really nice defender , he also have to score goals?... I think there are really good defenders that can lead an entire defensive unit who would be a really nice addition to a MLS team without scoring a lot of goals.... damn MLS teams pays DP money to some forwards that are outscored by Eddie Gaven for gods sake.
No, I don't think MLS should pay decent money to a defender. And, if your DP forward is outscored by Eddie Gaven, you threw away a lot of money on your DP (and I've always liked Eddie Gaven).