User blog comment:Wrath022/Chapter 517: Wendy Belserion, Review/@comment-4800978-20170115122006

I haven't really worried about Wendy's enchantments being comparable to Irene because, well, until Irene we hadn't had a comparison to judge the strength or effectiveness of Wendy's enchantment magic with. Now we do, and it just gives us renewed value in Wendy's supporting magic.

And I guess this just highlights the real difference in their enchantment magic - their application. Wendy uses enchantment primarily to support her friends, while Irene uses her enchantments for brutal offensive. Each application suits their personalities.

As for why Irene made the switch in the first place - because I've seen people complain about that - her personal goal of obtaining a human body superseded her commitment to Zeref's war. In that moment she didn't care about winning or losing, she just wanted to 'live' again.