User blog comment:FairyRaven/Fairy Tail Chapter 429 & 430: Code Blue & Purification Ritual, Review and Commentary/@comment-86.70.179.234-20150420104308/@comment-7418318-20150421081043

Actually that makes a lot of sense to me. She already merges part of the Spirit with herself, so merging with one completely could be possible.

With Celestial Spirits being under a constant pull by their world (which has to be counteracted by supplying magic power to keep them in the material world), I can easily see that merging with a Spirit would be a big strain on the body of the Celestial Spirit Mage and thus only useable for a short time without causing damage to the Mage.

Either that or keeping the merging up for a long time could carry the risk of permanently changing the body of the summoner, maybe even to the point where they could be transformed into an actual Spirit and thus become unable to live in the material world.