Stand-alone Google docs and sheets apps vs Quickoffice is the addition of the stand-alone Google docs and sheets apps an approach from Google to remove Quickoffice from the base Android build? What is different in the Stand-alone apps vs. the existing Quickoffice?
From self testing of the Drive, QuickOffice and the new Docs apps and an official Google blog posting, it seems like the strategy will be:


Google Drive app to view and organize your files
QuickOffice to open MS Documents
Stand Alone Google Docs apps (Docs, Sheets, and eventually Slides) to open Google Doc Files


Some testing I did shows (Moto X 4.4.2 with Docs v1.3.144.12; QuickOffice v6.3.1.041; Drive v1.2.563.21):


The QuickOffice app ONLY Opens Microsoft Format files; I am unable to load native Google Documents
The new Docs app ONLY opens Google Doc Word files; I am unable to load Microsoft Format files


A Google Blog post has a statement that leads me to believe their strategy is to remove document editing from the Drive app itself, and just have it open the stand-alone apps for Google Doc files, and QuickOffice for MS document files:


  If you don’t have time now, over the next few days you’ll be prompted
  to download the apps when you go to edit or create a document or
  spreadsheet in your Drive app. And of course, you’ll still be able to
  use the Drive app to view and organize all of your documents,
  spreadsheets, presentations, photos and more.


We also may be in line for a new Google Drive update soon - which removes in-Drive editing. The iOS version of Drive has stripped in-Drive document editing (in favor of the external apps) and added a Pin Code to allow you to encrypt local documents. (Source)

Edit: The new Google Drive app has been released, which removes in-Drive editing, and will prompt you to download the new Docs and Sheets apps.