The big issue I have seen is that the impact of properly selected magic items is enormous. A magic item melted down for residuum* is worth only 20% as much as one that is actually used, so carelessly picking a magic item that will not be used is like accidentally giving out 200 gp when you meant to give out 1000 gp.
So if your group has two characters in leather armor and two in scale, it doesn't really matter what level the chainmail you give them is, or what cool abilities it has. Oh, one of the characters *might* trade down to chainmail just to get the cool abilities, but even if they do, you can't count the "value" the same way -- it's a net gain, but probably not a very large one.
You also have to think about the items being replaced, and what will happen to them. It's a huge step up if the +3 Magic Longsword goes to a character who was using a +1 Magic Longsword, but not so much if the character was using a +2 weapon with really good properties. Also, does the old weapon gets disenchanted, or is it handed over to another character for whom even the old one is an upgrade?
This isn't to say that you can't give out items that you know the party will disenchant; it's just that you shouldn't count it as a full portion of the parcel.
Think of it this way -- handing the group a magic item that no one will use makes you like the grandmother who gives her grandchild socks and an 8-track tape by one of those groovy rock and roll bands that she found in the attic. (The tape was in the attic, not the band. Unfortunately.)
* Residuum, not residium. Pronounced as it is spelled, as everyone who plays 4e should know by now.