Lately, I've been hearing a lot of chatter about which tools web professionals should use and why. I'd like to begin by saying that I've never been one to drone on about tools. My perspective is you need to choose the right tool for the job. How do you ascertain which tool is the right one? The right tool is the one that does the job.
Let's take an example from the painting world: Bob Ross. I remember being a small kid, watching Bob Ross create beautiful paintings on PBS. Later, I re-watched those programs and found that he was not using a fancy set of expensive brushes. But rather, he was using a standard painter's brush — something most people own or can easily and cheaply find at their local hardware store. In 30 minutes at a time, using a big bushy brush, he created some pretty great work and a following of folks who suddenly realized that painting was accessible to them.
So back to tech, web designers have been offered various tools to create designs. Back when I was really young, there was Paint Shop Pro. Then there was this better more comprehensive program called Photoshop by Adobe. But wait, there was also Fireworks by Macromedia (which gained some speed because there was this new player on the block called Flash that everyone was starting to become familiar with). Later, Adobe gobbled up Macromedia and the debate has shifted to which Adobe program is better for web designers to use: Photoshop or Illustrator? (Sorry, Linux users, no one talks about The Gimp, although it should be addressed that the open source option exists and can be a viable option as well.)
My thought on this matter is: who cares? As a designer of websites (but more so a developer at heart), I know first hand that neither Photoshop or Illustrator will actually give you a good idea of what a website will look like. The fonts won't render exactly the same as in Photoshop or Illustrator (certainly not in every browser), images will need to be optimized for load, containers will shrink and stretch to accommodate the content. And at the end of the day, the experience (design/functionality) needs to be in synergy with the content. In fact, this is why you'll see so many things launch in beta. User flows, wireframes and design mockups can only take you so far.
I just read an article today that I really agree with from a firm that chooses not to send clients PSDs as the final deliverable, but instead a coded page or two. It sort of informs this idea. The idea that Photoshop or Illustrator is better or worse isn't the point. Your PSD (or for Illustrator fanatics, AI file) is a concept. The HTML coded from that is the proof of concept. Proof that this concept can be executed on the web in a semantic way; works with whatever content management system the client uses; serves all the business needs (good SEO, functional in a mobile environment, etc).
As such, I feel that web designers should not feel the need to choose between children (Photoshop and Illustrator serve a lot of different needs; pick the tool that serves yours). When really, what makes a designer really GREAT is the attention to this proof of concept. The real tools needed are in your head: have some familiarity with HTML; enough to say, yes, what I'm designing can be built. Also, have some familiarity with the concept of content management systems. Realize that just because it can be built as a freestanding HTML document doesn't mean it will work in a land of templates (as CMS-es often are). Finally, be understanding of limitations. And realize that limitations are going to vary depending on the environment you are working in, business needs, the target user of the website (as this user will inform user interface design choices) and etc.