Having W&B in the product is not a big deal to me either way but I'll stick my nose in here because I feel that you're wrong in saying that iFly is just a "navigation tool". It's EFB (Electronic Flight Bag) software. It is a "flight tool", not just a "navigation tool". For instance, online filing of flight plans has nothing to do with navigation. It has to do with flight planning. As does weather. And briefings. Doing Weight & Balance is just another flight planning task.
You are right in saying there are other apps that do this. A somewhat-similar flight task that has been requested to be included in iFly is checklists. There are apps that do that too. I can see both sides on this isse. On one hand is iFly trying to avoid feature-bloat, and adding stuff that would not be best-in-class compared to other apps that do that one feature well. On the other hand is the desire of users to not have to have more apps than necessary.
I personally think that W&B is probably a good idea, would not be that hard to implement, and would give additional (optional) data to Brian for calculating a Glide "Ring" (sorry, that reminder just snuck in there).