If a person who is leaving REALLY wants to be heard, they can take the time to construct a well-worded, non-pissed-off-sounding essay on the official forums with their reasons listed in an organized and easy to follow format, in addition constructive ways to either change the issue or reach a compromise. Problem is a lot of people just want to flame the hell out of Blizzard in that form. If you take to the forum and really think before you type, you might end up saying something profound.
You don't need to post it in an automated form, because those things don't get read most likely.
I don't usually leave because of changes made to the game. Though, the last time I quit, I wish I could have given them a well thought out essay on why they need to make single months purchasable, as an addition to their current offerings. We managed to do that back in October (which may have been a system bug), and it worked out SO well for us. Problem with the monthly subscription model is that not all of us get paid regularly enough for that time of the month to not fall between paychecks and cause issues elsewhere. It's just be nice to let the account freeze long enough for that next check to come in that has enough breathing room to pay for game time.