Fai said:
1. Regeneration and hardening can't occur at the same time.
2. Wrath is capable of moving faster than Shield grows.
Thus it makes more sense to just put the shield to good use as weapon and retain the regenerative abilities, without risking of getting into the same situation as in previous fight.
Just to butt in here, Wrath and his eye would have prevented Greed from getting into full shield from when we saw him in the scene. Greed could have done it before the scene, but he didn't. Besides, Greed has Ling on his side now, so he must feel he can go toe to toe with him (maybe why he didn't transform in the first place).
As for this chapter, definitely a set up, a lot happened, and it was all great.