Photoshop mock-ups for web designs are very common, and are also one of the reasons why businesses complain so often about not getting the results they wanted once their websites have been completed. Mock-ups are commonly used in a custom design situation with the intent to show a graphic version of how the final product will appear, or to give a client a choice between multiple design options in a graphic format.
There are multiple disadvantages in providing Photoshop mock-ups for web design clients. Photoshop mock-ups are expensive and time consuming. They are not made to scale, as the mock-up design may or may not be compatible with the final system being used to implement the design (html5 or one of multiple CMS platforms). A Photoshop mockup shows none of the functionality of a website design; i.e. jquery sliders, RSS feeds, etc. Not enough detail is displayed, and it is difficult for clients to understand the concepts with a flat mockup rather than in interactive html site.
Additionally, a very complicated Photoshop mock-up, while attractive as a static image, may or may not be easily translated into a final functioning coded website. This issue crops up most ofen when a designer, someone skilled in Photoshop, is not a person skilled in CODING a website; their mock-up design concept in Photoshop may not translate well to the web, and the final site may not completely duplicate their original design concept. And while we do have those clients that insist that their website duplicate pixel by pixel the Photoshop mockup they present us with, they are made to understand that this kind of time and effort comes with a price. A steep one.
In the same amount of time that it takes to design and create one mockup in Photoshop, an actual first working draft of a functional HTML site can be designed and developed for a client's review. A "live" site can be tested and reviewed for functionality and cross browser/device compatibility as well as looks. It can be displayed on a development site for edits and changes instead of wasting the client's time and money on flat Photoshop mock-ups. What is the point of editing and re-editing (and re-editing) an image (mockup) when you can review and test an actual website and make any changes there? You are more than one step ahead in the process.
We have many clients that come to us with a PSD that has been provided to them by a graphic designer, and it is our task to take that PSD mock-up and create a functional working website. This can be successful as long as there is communication between the client and the web designer, and the understanding exists that not all mock-up designs will look identical to once coded into HTML or Wordpress or the CMS of choice. Not being involved in the editing and re-editing and re-editing of that image cuts down on our time and our costs, but eliminating that part of the process completely saves everyone time and money.