When someone says “I don’t disagree” does that mean they agree?  Why not say “I agree”?  Are they not sure that they agree and maybe they disagree?  So why not say “Maybe I agree”? 

If it got to a point where they weren’t sure if maybe they agreed or disagreed, would they say “Maybe I don’t disagree”?   

I’m not sure I agree or disagree with this conversation approach, so of course I googled it. According to Yahoo:

"Saying 'I don't disagree' doesn't necessarily mean that they agree.  They could have no opinion, or be neutral on the subject, or agree.  Though, if someone does agree, they should just say 'I agree'."
 
Personally I don’t disagree with that explanation. 

At the Urban Dictionary, commenters had this to say:

***** 

“Essentially means, ‘I hear what you're saying, but I still think you're full of shit.’

“One agrees to the other's comment, but not really, in a passive-aggressive way.”

“Often used in the work environment to piss off co-workers:
“Perky Employee:   I think we should get a work group together to determine how to build company spirit. I really think it would build morale.
“Co-Worker: I don't disagree with you, but I think comp-days would work better.”

***** 

Actually it’s hard to know exactly what to make of this troublesome, passive-aggressive expression.  Clearly many people like to say it, and others don’t.  Probably the best we can do is agree to disagree on the matter.