Generic or custom layout?
Layout is one of your most important design decisions while creating a website. It is bound with proper understanding of what needs to be done (customer requirements), what the site will contain (type of information and its organization), graphics, proper font and color scheme.
Here I'll try to point out the advantages and disadvantages of having a generic and custom layouts.
Many web design companies offer ready templates online. These inexpensive layouts allow customers to build up a site almost immediately. They only need to apply their content to the existing layout. If price is crucial or the site is not for serious purposes, that is a good way to quickly provide a content to a large audience.
Many companies use free solutions to attract customers. WordPress is one such, widely used system, which is easy to use and quite popular. Billion websites use its platform, which has made them practically indistinguishable. Free or cheap solutions are rarely widely supported. They are inflexible and could hardly be used in case of changing requirements. You'll mostly need to begin from scratch, because the template customization time may not be justified.
Custom layouts do not rely on ready templates. Their timing is more accurate and may make a website unique on its own. Such layouts are created for specific customer needs and are therefore more suitable for serious purposes. Their flexibility allows them to integrate more advanced futures (such as content management system or payment system), to emphasize on corporate identity and to achieve more polished look considering even the smallest details. A disadvantage is that they are developed at a slower pace, and may require sometimes months to complete. Because they are consuming more time, the customer is charged much more than for a generic layout.
Why not use them both?
If you have a client, who needs a custom solution without investing much time, you may still want to buy a ready template for him and customize it on your own. That will cut the costs for the client and sometimes minimize your delivery time. In practice this is rarely done. I think one of the reasons is that generic layouts may undermine the work put in customizing the template. Another factor is that the designer needs time to figure out how to best use the template. Depending on it's complexity, he may need to invest time understanding and properly using the underlying code.
I have decided to use only custom layouts on my sites.