Why should it take that long to drop to all fours when you can grow a dozen limbs easily?

Keep in mind this flexibilty is coming at the cost of having a spellcaster. You don't have a summoner to buff you, or do battelfield control, while you go tear things up.

Also, limiting them to 1/2 their points isn't really going to stop themfrom doing much. even 4 floating points is enough to let them gain fast healing at will, throw up spell resistance, gain blindsight, gain Dr, gain a burrow speed, become immune to any damage type, grow wings, add energy attacks, etc. That is more a limit on what flavorful transformations they can pull off than actually limiting their effectiveness.

Also, these changes require an investment in their action economy until very high levels. And at those high levels, spellcasters can have even more flexibilty if they want . A spellcaster with shapechange(happens at 17th level, compared to the shapeshifter's 15th level for swift change) gives them access to most of these options. At low levels, giving up a full round or standard action to change means you are not contributing for likely a large portion of the battle.

Its certainly powerful, but does it really compare with double the action economy with a spellcaster, who could force most of the situationally useful changes anyways, can buff you, summon reinforcements, and at high level's duplicate your combat ability? Does this flexibility really match up wth what mages can sling around? The resitances/immunities can easily be gained by a single spell, spellcasters virtually alwasy have some form of fly available, burrow is a low level spell, stoneskin grants the DR, the breath weapon is comparable to an evokation spell...

I considered haivng a rounds/day limit on being shifted ,liek a barbarians rage or a bard's music, but while a non-raging barbarian can still swing an axe and a non-singing bard can still cast spells, a unchanged shapeshifter is just naked and useless.