Highlighting sentences in the statement of purpose
I use a similar approach, using bold to highlight key words or expressions in a short free text summary in my CV. The benefit is that points I think are key to my career are visible even at a glance. As you mention, many are stressed and reading through letters take time. Although the persons responsible for the admission process should read the material carefully, it is only human to, for example, get tired after a large number of applications. Using highlighting and other forms of structuring elements (paragraph breaks or even bulleted or numbered lists) to make the text easier to access is therefore a good way to make the reading easier.

To anyone thinking that highlighting might induce unfair advantages, it is worth stating that it can also easily back-fire if the highlighting is not consistent or give an impression that is not consistent with what the text or other supporting material will indicate. In other words, any highlighting must be done with care to be a tool to simplify the access to the information and not to skew it.

Depending on where you are in your career, the CV will be the most important part of an application. Finding ways in which to summarize, for example the general impact of your articles or other activities becomes important. This is why I have a short summary text at the start of the CV, the lists of publications is then supporting the statements about number of publications, the citations of individual articles, etc.