You should always use the app that does the best job for a certain task. Whats important (to me at least) is to achieve the best result possible, not to use as few apps as possible (even if that involves a couple of extra steps). I'd be great if you could use just one app for everything, but that won't happen because all developers focus on different areas of the workflow, and thus the apps have their strengths and weaknesses in different areas.
Dedicating development resources on the 3DCoat rendering engine for example will produce a better rendering engine, but it will be inferior to other already established apps, simply because the other developers have dedicated resources on that area exclusively for years.
I own both 3DCoat and ZBrush, and use both. I do sculpting in ZBrush, but for certain tasks 3DCoat is better/ faster (voxels).
I do texture baking in Marmoset Toolbag or Knald, Retopology in Modo, painting mostly in Substance Painter, somethimes in 3DCoat. I could do all those things in 3DCoat, but I'd miss out on a lot of great features in the other apps that produce superior results in the end.
Its often better to use specialized apps that can do a single task best, than to try and do everything in one single app. That might work for hobbyists, but not for professionals that need to produce models of the highest possible quality.