If I understand you correctly you are saying we should not touch the design of a button because it could confuse the user, that would mean every website would have GREY buttons.
I personally think you can style the buttons as much as you like, as long as they are CONSISTENT throughout your site.
I think the point being indicated to the user in using the default button is that this is a button that operates like any other button on their OS. On Mac OS X for example default buttons are a recognisable blue as are other form elements therefore a user (and we mustn't forget that many users do not distinguish the actions of a web page in a browser from those in any other application on their OS) has an expectation of it's functionality. Note that functionality includes all the states of that element as well, such as on focus, on click, etc.
One of the problems I find with "designed" buttons is they tend to be biased towards the OS the developer knows. For example seeing XP like buttons on a page viewed in OS X is not intuitive or particularly attractive. The main problem is that to change a button and maintain consistency you need to change the styling of other elements, such as a drop-down, as well.
There are of course good reasons for either solution and it's true that the main importance is consistency. The question is what is "consistent" for an average user: the look of the buttons on a single site v's other sites v's the ones on their OS? Also, not to be disregarded; who is the end user of your site, savvy or non-savvy, and how much does this allow you to play around with their computer using comfort zone.
Nick
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