A handful of lines of JS that runs the Luhn checksum on the number in the "credit card number" field is not at all CPU intensive and can save people from such data entry errors.

If I typo the card number and there's no JS code to tell me that, I need to submit the form, wait for it to error out, have to re-enter all the billing address information, re-enter the card number, and try this whole process again. I've resorted to re-typing the card number in the "find" box and making sure that it matches the number in the form's field, just to avoid this crap.

That doesn't necessarily have to be the case. If done correctly, the page could come back with errors and the form already filled out. If the connection is secure enough to fill out the form - it must be secure enough to return your data you entered.

I think javascript is a nice addition to client side validation of forms, but it must not impact how they use the system. Having a piece of JS run and not allow me to submit a form is really terrible design, especially when the validation it is doing is running incorrectly.