Most small business people aren't going to go around installing new designs on their site on a whim (isn't that what they paid the designer to design?).

They want something they can log into once every couple of weeks and post some minor update too, then possibly consider a redesign 2 years down the line at which point they are likely to want to throw out most of what they have anyway which doesn't matter since they spent maybe $1000 on the whole thing.

If the site provides any kind of complex functionality then wordpress is really no longer going to make sense as a core to build around. Here 90% of the site is likely to be customer forms, order processes etc that don't fit into any convenient pre-existing model. At this point if you build around an off the shelf CMS 90% of the site will be custom plugin code so there will not be a huge benefit to anyone who would take over the code.

It's all about making the business itself a first class citizen rather than a particular piece of software.