For me it's about ease of use. It's not that necessarily I am looking for MORE than the built-in can do (although that's handy for when you do need it!) but it's mainly about the SPEED of use.

A big screen and a piano roll display often makes it simplicity itself to visualize what's going on, to easily spot where things went south, and to quickly see what needs doing. Plus there are usually key combinations to do stuff like quickly select all the instances of just ONE note (say you want to select all the ride cymbal notes to raise or lower them) without having to type into fields, etc.. As you say, when laying things down quickly, you want speed. Thing is, I can do things ten times faster on a computer!

AND... when I've finished the MIDI editing part, the project is already in the DAW for laying down audio tracks to, or replacing some of the parts with VSTi's, etc.

One of the things that makes using a computer so easy for me is that, at home, I set up my keyboard at my computer workstation, with the nearfields wired in, ready to go. I've got something very similar to this: http://www.walmart.com/ip/Sauder-Saturn-Multi-Level-Computer-Workstation-Black-and-Cherry/11003066 and with the keyboard above the mouse and keyboard drawer, the monitor above that and the nearfields on the outside, my arranger isn't just connected to my computer... it's a PART of my computer.