How many times pe day do you need the level 2 or 3 spells? A caster can get overland flight, the defensive stuff are all duplicated by trivially low level spells. A summoner can give any of them to their edilon at a moment's notice without lowering its power. A shapeshifter has to stop, gain the ability it needs, and sacrifice something else its doing. At low levels, its pool of evolution points is very limited. If it uses one of the defensive powers, they will severiously cut into their offensive capabilities. By the time this is less of an issue, all of the abilities are things spellcasters have trivial access to. At first, it takes a full round action to activate them, and they are sacrificing a large portion of their power to it. At higher levels, it takes a standard, which puts them on par with a caster for activating it, but it still eats into their other capabilities. At this same level, a summoner's eidolon could be fully tricked out for combat, be attacking, and still receive these bonuses. When they get to shifting as a move, they get a slight advanage over a caster, but by this point they are truely low level spells for a caster, and its still a significant sacrifice in actions. If you are next to the enemy, you just dropped your full attack(which they should be really good at) to a standard. If you are not next to the enemy, then you can't attack at all. This is not a trivial tradeoff in combat. At very high levels, (15+) they finally get to shift as a swift, giving them an action advantage over caters. Unless the caster's quickened them, which is a low level spell at this point(6th level spell when they have 8th). And the summoner can still be casting spells while their eidolon full attacks, and you just have yourself.

I could easily build a sorcerer with all the flexibility of a shapeshifter, and I wouldn't feel that I had made a poor spell selection. If it was a bad spell selection, then its not a problem for the shapeshifter to have it. Generally speaking, you don't need any of the special abilities more than a couple times a day. You don't access any of them before a caster would. You have an extremly limited set of abilities to invoke, at significant cost on the battlefield.

I'm not saying its not a significant ability, but it doesn't compare at all to what casters can do. Most of the shapeshifters abilities are direct combat improvements, which they get instead of class features.