User blog comment:Gourgeist/Shingeki no Kyojin Chapter 66 Review: Wish/@comment-7418318-20150210234336/@comment-5433366-20150211002650

Perhaps it's like the key to the basement, it's not really necessary, and only for symbolism and "cleanliness" (as in, he wouldn't just break it and make her lick it off the ground, lol). They may be a specific about needed to transform or something. Who knows how much Rod has hidden away anyway, he may not be concerned with the amount wasted when injecting a person. Or it may just be an unconsistency on Isayama's part.

Btw, he's just a regular Titan. He has to devour Eren in order to become a Shifter and gain Grisha and Frieda's memories.