Should I Kerdi or not?
If you drywalled a wet area then I would consider Kerdi a must do. If you used cement backerboard then I'd say it is an option, but some sort of waterproofing layer between the board and the tile is a must. Redguard is a cheaper option, but not on drywall. Drywall under tile in wet areas is a code violation in many, many areas nowadays, for good reason. Kerdi can be applied over drywall and will render it as waterproof as a frog's butt. Without Kerdi your only option is removal of the drywall and re-hanging the wall with wonderboard/durarock, then application of Redguard or similar paint-on waterproofer.
If you hang tile straight to drywall in a shower/tub area, you're guaranteed to have to re-do the job within a few years. Not to mention the moisture in the wall.