User blog comment:IamJakuhoRaikoben/Chapter 433 & 434: Ikusatsunagi & Demolition Fist, Review./@comment-1776824-20150512000613/@comment-7418318-20150512072145

Didn't both Mard Geer and the Spirit King himself say/think that the CSK couldn't remain in the material world for much longer and that's why he used Galaxia Blade as a last resort before he was pulled back into the Spirit World?

At least that's how I remember the scene.

But your other points I agree with. When Lucy summoned something that was close to or even beyond her limit to call, those Spirits were very powerful, but could only fight for a very short time.

Good examples for this would be Loke fighting at full power against Bickslow or the already mentioned Celestial Spirit King. Lucy's lack of magic-power limited the time the Spirits could keep fighting, but didn't seem to weaken the Spirits' fighting abilities.