From the questions posed in another forum that I contribute to it would seem that designers who use Wordpress fall roughly into two categories...
(A) Those who have to design for clients who have purchased, or already have, a Wordpress template and need to have it changed to suit their needs.
(B) Those who have no clue about web design and are trying to pass themselves off as web designers.
The main reason for using a CMS is to allow unskilled (cheap) labor to update sites and add content. Most small businesses don't update their sites on a daily/weekly or even monthly basis so why would they need a CMS?
A CMS would normally be used for ecommerce sites that have more than a few products and where the owners are adding to them regularly. If this is the case then they would be better off using a dedicated ecommerce solution like Magento which has a free "Community" version for those on a low budget.
One of my recent projects was to design a mobile site for a large manufacturer of health products. My part of it was to create the design and layout of "template" pages for the different product categories which were then imported into their CMS so that the webmaster and his staff could add the individual product descriptions etc. The main work was in developing a swiping slider with popup pages for extra product info.
If you multiply the number of products by the number of individual pages required for each one plus the category pages, movies and all the other stuff, this website contains several thousand pages. Without a CMS (and somebody with the patience to use it) it would take a long time to create.
I doubt if too many iWeb users run sites of this size! The company did get a Wordpress style designer to attempt this but the result failed to impress the chairman when he tried loading on his iPhone. Apart for that, Wordpress didn't have a plugin that would work for their desired product display method.
Another reason why "designers" use Wordpress for sites other than blogs is that they need login facilities and/or a search function but don't have a clue how to do this themselves or are too cheap to pay for a decent solution.
The profusion of available plugins is another attraction for those that can't implement additional features themselves but they are not exactly cutting edge. Having said that, Wordpress is a lot better than the online site builders in this respect.