Kimberly is spot on, especially with the part about the unnecessary code that clogs some templates. I used a free Microsoft Office Live template for nearly two years, and I hardly got any search engine traffic to my website, despite my attempts to optimize my site. I assumed I was just awful at
SEO, but since I've had my new Wordpress site up, my rankings have increased.
Plus, a custom-designed website just looks more professional, in my opinion, and it helps to set you apart from the pack. And you can also tie your website in with your brand image to help give you an even more impressive look, in addition to building the know, like, and trust factor (if you have a website that looks one way, a newsletter that looks another, and advertising that is completely different, no one will ever be able to recognize your brand from your competition, so there is no chance of building brand loyalty). This may sound like something you don't need to worry about as a service provider, but if you put the work into getting your name out there, you want to get the credit...and the resulting business!
Of course, as Jules noted, I'm a bit biased too, being part of a web design team that offers customized Wordpress websites!

But I do also speak from personal experience, so take that for what it's worth...