What she is getting at is if an individual in your class is one or two pieces away from completing their Tier 1 set, to please pass (at your discretion) on the piece so that the individual may complete their set. The reason being is that most of the Tier 1 sets have very beneficial 8 piece set bonuses that help the raid considerably.
The other thing about MC, BWL, and soon Naxx (although the armor comes from quest there) is that the set pieces are geared for that instance and the beginning of the next. You help yourself and the raid by getting the complete set vs. passing for other items. There are the set bonuses and resistances attached to the Tier 1 and Tier 2 sets that are invaluable. Outside of the Raids itemize however you see fit. In the 40 man runs where this stuff drops in most cases you are going to help the raid be more successful with the set. All casters (druids and shaman included) get mana / 5 on most pieces of the tier 1 set, and it's needed for the longer boss encounters. Some also help with dropping aggro and tanking.
Ultimately it comes down to how you want to spend your dkp, but if you look at it from an overall raid standpoint, and from watching other successful MC/BWL/AQ40 and soon Naxx raid groups, they are all in their Tier 1/2 sets for that particular instance.
Anyhow...that's what she was getting at along with my personal thoughts and observations. Once again, no one is telling you how to spend your dkp, just asking if you know a member of your class is close to completing their set to pass. You don't have to pass, and won't be penalized in any way for not passing.
That's a good point, and I agree with you. Maybe Maube will come in and clober us or give us a good answer on that!