Of course that at least objects of white quality should be possible to transmodify. In fact, purely aesthetic objects designed only to copy their model should be white-quality (actually, most of aesthetic objects that are sold by NPCs are of white-quality). The excuse that Blizzard puts is that there are some white-quality items that have not appropriate models... For the love of God, limit only those cases! but not all of them!
In my humble opinion, most transmodification restrictions should be abolished, if not all. We have to have complete freedom to alter the appearance of our character! We should be able to copy the appearance of any object that we can equip!
And as an own suggestion, I would like that the off-hand items could be transmodified in main hand or two-handed weapons and vice versa. That could lead to interesting combinations ...
Let take a practical example, imagine that I want to equip myself with a Kirin Tor style, and for so, I would need the following items, which, with the existing restrictions, are impossible to transmodify nowadays:
Head:
http://www.wowhead.com/item=44647
Chest:
http://www.wowhead.com/item=5110
Main hand:
http://www.wowhead.com/item=44638
Off-hand:
http://www.wowhead.com/item=44655
But I'm afraid we can only dream ...