Salesforce Lightning: Setting a conditional default value blocks any further changes to the attribute.

TestComponent.cmp

<aura:component implements="flexipage:availableForAllPageTypes">
    <aura:attribute name="myString" type="string" default="{!if(false, 'Default Val', 'Other Default')}"/>
    <aura:attribute name="mySimpleString" type="string" default="Default Val"/>
    <aura:handler name="init" value="{!this}" action="{!c.doInit}" />
    <div class="slds-card">
        {!v.myString} <!-- Will always output 'Other Default', even if changed later by doInit -->
        {!v.mySimpleString} <!-- Will be changed to 'Set Directly' by doInit -->
    </div>
</aura:component>

TestComponentController.js

({
	doInit : function(component, event, helper) {
                component.set('v.myString', 'Set Directly');
                component.set('v.mySimpleString', 'Set Directly');
	}
})