Say I have a DS575 E. It is a 24 channel fixture and I address it to channel 1. I program a scene with it and then I need to change the address to channel 101. Instruction on channel still keeps the 512 channels on the universe the same instructions, so I have to reprogram my scene. Instruction on fixture has to move and assign all 24 channels of that fixture and then empty the 24 channels where the fixture was originally addressed. While it does this, it must update the scene instructions as well for the new address. Not exactly a simple task. Well, imagine that with say 10 DS575 E's or 100 scenes or even worse, 100 cues or even worse still 100 cue stacks with 100 cues a piece. It gets real complicated real fast.
With regards to designers only using consoles and not software based controllers, the biggest reason is dedicated to the task. A PC has to do much more then just run DMX software. Now the PC has to do much more, just look at the above. Since it is also dedicated software and hardware on the console, less comparability issues and conflicts. It is a much more controlled environment. I can tell you that my Road Hog has an Asus motherboard, an Intel P4 2.8 ghz processor, a 40 gig Seagate hard drive, and 512 mb of ram. By today's standards, a below average PC. Most have dual core processors and at least 2 gigs of ram with 300+ gig hard drives.
The other major advantage to having a console is the interface.
Having all that at my finger tips is much better then trying to to get a mouse and hit all that. It would be like having a cell phone with a mouse and to text, you need to use the mouse to enter letters. Doesn't work real well. Likewise, the touch screens make things much quicker as well. Instead of having to move the mouse across two or more screens, just simple touch and its there.
Those are the two major things with regards to consoles vs software controllers. There are other advantages but those are the two major ones that come to mind.