An end-user may not notice or care if you stick a form class on your form element, but you should. You should care about bloating your markup and slowing down the user experience. You should care about readability. And if you’re getting paid to do this stuff, you should care about being the sort of professional who doesn’t write redundant slop.
Tim Baxter in 'Meaningful CSS: Style Like You Mean It' – A List Apart
I've had this argument with colleagues a lot over the years. Attention to detail is important. The errors that come from a lack of attention to detail are what used to separate professionals from dilettantes. It doesn't matter that most people won't notice. It should matter that you know the difference between excellence and slop and that you reach for the former and are ashamed of the latter.
It's why having standards is so important.