I find it interesting that despite how powerful echidnas special birthing version of planar binding is, for role play reasons, she can't use it for one of its most classic usages. Creating body for powerful animated Undead. Outsiders because they're high power / hit dice usually make great Undead and you can just bind them into a trap then animate them. You can do it for other party members even someone that has no optimization on planar binding whatsoever can get a marilith or pit fiend at our level.  However echidna is not really going to be keen on that idea.