Removing the dotted border on active links in FireFox...

I was involved in a discussion earlier today about the dotted boxes / borders that appear around links in FireFox once clicked. The initial question was how to remove them which simply involves adding a line of code to your CSS similar to this one:

a:focus {outline: none}

However - the topic soon turned to why they were there and should we be removing them. The argument was that they provide accessibilty for user that need it and should not be tampered with. In my eyes removing them is no worse that setting the size of text to a small font - it shouldn't be done but many, many sites do it regardless. Designers should be mindful of usability of their sites as well as the look.