The behaviour wasn't what I was expecting and I need to digest the official MS response. My initial thought is that the naming is "wrong" for what it does.
And since I started to blog about this, that definitive answer has been smacked down. I've started working on an example that I hope will show what putting a variable in readonly vs readwrite does but so far, I can't make SSIS show a difference. Win XP, SP2, 9.00.3042.00
Rough attempt on my skydrive