A work environment is different.  The problem is that you need a computer 
and skills to get a job.  For home use their is no problem with a free 
screen reader.
Almost everything we use is not based on the screen reader, it is the 
operating system and the applications.
A blind computer user should be easily able to transition from one screen 
reader to another.  Not seamlessly but with in reason.
Your employer chooses your screen reader in most situations, and you have to 
adjust to their choice.  Its not always a good choice.
If you want to access a computer in say a library for example, you have to 
use what they have.  They might let you load your screen reader from a thumb 
drive, but that is doubtful.  It won't pass the windows 7 security dialogs 
anyway.

How long has system access been around?  The same for NVDA?  Their is no 
guarantee that GWMicro or Freedom Scientific will be around next year 
either.

If Twinkies and hoho's can disappear, you can't bet your life on a screen 
reader.