This is my first bug report, so I will try to be as concrete as possible.
GAMES: BG1Tutu / BG2:ToB (Patch version 26498) (No mods installed)
OBSERVED BEHAVIOR: A shapeshifted character (via Polymorph Self, Shapechange, Druidic abilities, etc.) caught in Dispel Magic will have their natural weapons removed (such as the fangs from the Spider form, the fists from the Golem form, the halberd from the Flind form, the claws from the Werewolf and other animal forms, etc.) and the weapon quickslot icons will revert back to whatever weapons you were holding prior to shapeshifting.
This will also remove any visible held weapon from the sprite itself. The Ogre form will be seen without its morningstar. The Flind form will be seen without its halberd.
Following that, any weapon can be equipped from the inventory while shapeshifted. You gain any Thac0 bonus, damage bonus and equipped abilities from the weapon you equip, but also keep the attacks per round given to you by the shapeshifted form. You do however lose the secondary effects of your previous natural weapons (for example, a Mind Flayer without the tentacles cannot drain Intelligence and a Spider without its fangs cannot poison enemies).
As such, this can result in some very weird scenarios, like a spider firing missile weapons at a base 4 attacks per round, or a huge golem going invisible and dipelling stuff because he "punches" with a Staff of the Magi. I haven't been successful in dispelling the entire Polymorph/Shapechange effect, however, even with a cleric 10+ levels above the polymorphed caster.
DESIRED BEHAVIOR 1: The natural weapons granted by shapeshifted forms should not, in any circumstances, be subject to removal by dispelling.
DESIRED BEHAVIOR 2: I am not sure about this, but I believe the Polymorph Self and Shapechange mage spells (and other such spells I may forget) should be completely dispellable, so that a successful Dispel Magic reverts them to natural form and strips them off their ability to further shapeshift.