Can anyone sell me on Shapeshifter?

IMO shapeshifter is the worst specialization in the game across all classes. If I am a mage why would I give up my ability to cast spells so I can change into a lame ass spider? lol even though Morrigan starts as a shapeshifter I would rather have her casting spells and curses which is more useful than another melee squad mate so I go entire playthroughs without her transforming a single time. So my question is: what circumstances is being a shapeshifter useful? Has anybody committed to a shapeshifter build? Was it fun?

For transparencies sake I will admit I play exclusively on easy difficulty so maybe it’s more useful on higher difficulties?