I agree with all the above.
Controller + software is by far the preferable solution for people who are comfortable in that environment. A piano modeler like pianoteq uses barely any computer resources and creates a sound that would convince most listeners. But that's the exception, most acoustic instrument in software form will be sample-based libraries, meaning you need quite big hard disks and a reasonable amount of RAM.
Pretty much all digital piano's have MIDI in/outs, so it can kind of serve the same purpose as a controller keyboard, but if you're always going to software instruments, then a dig. piano is a waste of money on: speakers, built-in sounds, and the general 'piece of furniture' style of build materials.
I had a digital piano for a few years. I found it great to quickly play on the build-in sounds, and then hook up the MIDI to use it as a controller keyboard for properly recording the composition. This was fine, but these are the reasons I changed to controller keyboard, maybe some are relevant to you:
-it's smaller, better for live use (same number of keys, but not designed to look like an acoustic piano) - you can get controllers in almost number of octaves you want.
-controller keyboard came with assignable knobs and faders, so I didn't have to get a separate MIDI device for that purpose.
-more sockets for pedals. I need sustain and expression pedals. The digital piano had 3 pedals but they were fixed/unassignable, and they are all on/off pedals (none could fade between values like an expression pedal).
-more sturdily built, sockets are designed to be plugged and unplugged constantly - one of my previous consumer keyboards quickly lost all ins and outs as I swapped them too much.
-I needed 'aftertouch' for a particular software instrument. no digital pianos have this feature, as far as I know.
Having said that, digital piano's have the advantage of being quick to turn on: no waiting for the PC to load, no software fighting for the same sound drivers... I really miss that freedom sometimes.