One of the biggest advantages to using a content management system such as WordPress is the almost unlimited access to different themes and functionality. Spend a little time and you can search through the different themes using WordPress’s extensive search options (search by colour, column, layout, subject and features).
Several such themes have built in SEO features, calls to action, slideshows and image galleries, giving you pretty much anything you need. You can also use child themes to make minor modifications and add custom functions and designs to your theme without having to disconnect your theme from any future theme updates.
With all this sounding as good as it does, why would you even consider creating a custom theme from scratch?
To start with you may have specific requirements for call outs, online marketing, social media integration, SEO, structured data, and content flow as well as very specific branding. With these requirements, building exactly what you need from the start can be much faster than searching through (and testing) themes and plug-ins. Also if the theme isn’t exactly what you need it to be, you then have to spend money on a developer to modify it anyway.
I tend to work with a lot of designers and agencies. They know exactly what they want and as such, I find it much more efficient in these situations to start with a clean, responsive theme such as Twenty Thirteen and use that for my base to build on.
In many situations the decision on whether to use a custom or downloaded theme comes down to budget. Many themes are free or have a very small rollout cost while developing your own theme can be a bit more costly. If you are hiring a skilled design agency or developer, they have the skills and knowledge to guide you in website best practices and can strategically develop exactly what you require.
To summarize, I think using a downloaded or low cost theme is a good way to get your website off the ground on a minimal to low budget. Then, when you are ready to take your website to the next level with online marketing, SEO, brand promotion, or incorporate an eCommerce element, it’s better to hire a skilled developer or agency to develop a custom theme which can best address your company’s evolving website needs.