There are many benefits and advantages to using CSS over straight HTML, images, or JavaScript. You're right that many design characteristics of CSS can be duplicated in JavaScript. The major weakness of JavaScript is that it is easily broken, doesn't fit with web accessibility standards, and tends to create cluttered code when used as a design tool. Some other CSS advantages are:
1.) Many HTML style specific tags like <font> are deprecated and so run the risk of being unsupported by browsers in the future.
2.) CSS makes your HTML much more accessible to people with vision impairment. Remember, the audible readers for the blind read your code and not how a human sees the page. This means that JavaScript, VBScript, and images are unreadable.
3.) Audible readers aren't the only tools that read your code. Search engine spiders also read your code. Scripts and images are useless for SEO. With images you can use alt tags but being able to use a header tag is much more effective for page rank.
4.) Many effects that were only achievable through images can now be achieved with CSS, even rollovers. This can open up your headers and navigation to the search engines.
5) You will have cross-browser issues with CSS, however they are minor compared to JavaScript browser issues. I love JavaScript and use the language when needed, but I will always use CSS instead if I can. Cross-browser adoption for CSS is much closer and getting better all the time.
6) JavaScript is easily turned off and many people mistakenly turn it off for security and privacy reasons. CSS is rarely turned off so your design is less likely to fall apart.
7) Even if your CSS doesn't load you can make your page readable without it. This is only possible in JavaScript if you double your work and code noscript areas.

I can't stress enough how much easier it is to edit CSS generated designs over JavaScript
9) If you use a linked-in stylesheet file, any images you use for backgrounds get cached once for the site. The images don't get loaded for each page as they do in standard HTML. For example, if you have a 50K background image on every page of a 6 page site, with CSS the total download for the site is 50K, with HTML it's 300K! Plus in a year if you get sick of your image/font/etc. you can make a change in one file instead of 6.
For more advantages of the XHTML/CSS combo:
http://www.khmerang.com/index.php?p=106Most importantly it will make your life easier to use CSS in the long run. Cleaner code is easier to maintain down the road and is less likely to have bugs. I wish CSS was around when I started out, I would have fewer grey hairs now.

Depending on the version of DW you're using it can be easier to implement a CSS layout.
I hope this is helpful. If you do a search or look under my other postings you'll find links to useful CSS/design sites.