The BG I & II series implemented shape change and polymorph magic primarily for combat activities. I loved the 100% magic resistance of the mustard jelly for mage/lich battles, but I rarely used the bear or wolf forms. Shapechange could be useful for other things that I definitely would use. For example, a squirrel or bat form would probably go unnoticed by most enemies, and could make an excellent scout. I imagine that could be implemented as having an automatic hide-in-shadows innate ability. Another possible benefit of a small creature form would be the ability to squeeze into small spaces, thus possibly providing the character with an advantageous position (think of iron golems stuck in doorways in BG II). Enhanced senses of a shapechanged form might help detect invisible creatures or illusions. Perhaps even water-breathing might come in handy, who knows?