To try and steer the conversation back out of a "class vs. job" war, I am going to post the pro's and cons, pointed out by people regarding the OP.

Pro's
- Elemental affinity would improve if the spells were simplified onto one job.
- BLM's would become a little more flexible to the situation, and overall mechanic would be slightly improved.
- WHM's would have access to their iconic divine spells in previous FF's, beyond Holy.
- WHM's spells would resist less, but be strong against less.
- All elements would be on an even playing field.

Cons
- Lore would have to be modified.
- WHM would lose elemental affinity of the standard elements.
- BLM would lose some skills
- The animations would have to be reworked.

Well that is the current gist of what everyone has been saying in here. If I have missed an obvious one, please don't hesitate to correct me and I will edit this post.

We can throw back arguments at each other all day and get nowhere, however a dot point list of pro's and con's is potentially a good start to making some form of middle ground.