Flotrol & XIM generally don't negatively affect coverage (hide) like water does. Also, XIM gives the added benefit of causing the paint film to level & flow better.
Flotrol is a paint conditioner - so it allows for better viscosity of the coating during application, and it also helps extend shelf life of leftover paint. XIM, on the other hand, is a flow & levelling agent - so its primary function is to allow the paint film to level out once it is applied to the substrate.
Personally, I use XIM in nearly every waterborne coating I apply. There are a few exceptions (like Aura) where IMO it is not necessary to achieve exceptional results... this is where the judgement call comes in. If you know you can achieve a nice, smooth, level paint film with a particular product, then it shouldn't be necessary to use an additive.