Well, nofollow doesn't mean that the comment will receive no link juice -- Google is the only robot that doesn't actually follow nofollow links (Yahoo! and Ask will still follow the link and index it. Yahoo! simply won't assign any "weight" to the link from the site -- so it's just like a regular link, not an authority link.)
That said, Linda offered some great advice. Commenting purely for traffic isn't necessarily the best way to go about it. You should be commenting to
network with the authorities in your niche and with your potential customers/site visitors.
Use the blog's comments as a way to get on the blogger's radar. Later on, they'll recognize your name when you're ready to move the communication to another level.
~ Teli