First off, welcome to the forums evader, and thanks for taking the time to discuss your views directly. And as you've likely already guessed, i'm going to disagree on your last statement..
Much akin to port-scanning, which is not illegal (IANAL) .. entering in non-normal inputs on a public website or wget-ing with query strings that usually never sent - is completely benign. If someone signs up for a new account name as MrSunshine!@#$%^&*()\/>< and it inadvertantly crashes your server .. then you may reason to claim downtime losses. However, searching for the string asdf'e"e>e<e changes nothing on your server (aside from access logs) nor does it negatively impact it in any way. To quote an old adage, 'No Harm, No Foul'.
My personal purpose for testing major sites I and others visit, is to raise awareness of how unsecure most every substantial website is. I am not taking into account the quilting site my grandmother visits http://mars.ark.com/~quilting/ .. From what i've seen, maybe 20% of major sites have no signs of filtering at all. Another 30% or so - yes i pulled these percentages out of my ass - filter the two main places Search and Login, and ignore everything else. That means registration forms, feedback messages, forgot password boxes, newletter signups, contest entries, etc, are all vulnerable to arbitrary injection. Most disheartening though, is that certainly fewer than 5% of websites i've checked have completed a once over with no exploitable vulnerabilities.
That is what gives me nightmares.. (that, or a mixture of vodka and Saved By The Bell reruns). To think, any website i visit could potentialy have a hidden iframe that uses CSRF to exploit an XSS hole on http://neopets.com and force me to send them my prized Kougra..
Isn't he cute~?
I don't know how i could continue on living, should that happen - not to mention my bank's website or my email host.
So yes, i am guilty of spamming the logs of many of those websites i posted about on http://sla.ckers.org/forum/read.php?3,44 with scary looking alert and <script> strings inside the page requests. Did i scare some of the admins into thinking their site was under attack? I certainly hope so. If 19 out of 20 of those admins have a hole on their site that a malicious person can take advantage of them or their users with, they ought to be scared. If they were competant, they could see where the pen-testing ended .. and locate their flaws. As a user myself, i'm sad to see that most every webmaster i entrust with my passwords and personal information, has no sense of web application security.
But back to your last statement, that we should only test your software and not your public-facing production site - i strongly disagree. The site that promotes the product is just as much their software as the product itself. Likewise, their is no download for my bank's website source code .. so i'm unable to test it on a test server in isolation. But i'll be damned if i take them at their word that noone can transfer 500$ out of my account everytime i visit their blog site.
The moral is: Don't hurt those who want to help you, and leave my Kougra out of this.
-maluc