Another one for space saving. Most houses also have the combined bathroom so its something we are used to having. When we purchased our current van we spent time considering how we were going to use the motorhome. For extensive touring without using sites, then a separate shower/toilet has its merits and the all in one has fewer. If using site facilities, you will be able to compromise on how much space you want to fill with the toilet/shower. Draining a shower can be problematic if the van slopes away from the plugholes. We use a large microfibre drying cloth to dry the shower area after use.
No matter what system you use for dealing with your waste products such as chemicals or a SOG type system, there will be odours from time to time. A separate shower means that you can shower without this consideration. Also, if there are more than two of you, then a separate shower may be better. Some people use their shower to store wet items until they dry - flapping your way through wet coats and washed underwear to get to the toilet is not for everyone.
Cleaning an all in one can be difficult because of the limited space but you only have to clean one area. If your svelte days are in the past, make sure you have enough room to undress, shower, dry, and dress. The more confined the space, the more knocks and scratches are likely.