More importantly, it's a bad business and ethical decision do do that without talking to the couple first. I don't necessarily agree that it's bad to put promo footage on YouTube, even if their encoding is bad. However, I agree it is not as classy as hosting the footage on your own site at better quality.
If you leave the comments turned on, what if people say bad things about you or the couple on the Youtube site for that clip?
What does it say about trusting you as a businessperson if you went ahead without permission and did this, and it got back to them? Wedding business is all about referrals from happy customers, they are your best advertisement. Why would you risk turning them into a continuous source of bad-mouthing by going behind their backs? You'd kill more business than you'd get.
Ask for permission, and be sure to update your contracts and sales pitch to raise this issue up front from now on, then give the clients the option. That turns it around from a sneaky rip-off to proactive added-value, where you're making a few key clips available for all their friends and relatives to see online. And it just so happens to also promote your business, instead of creating bad press.