I have had the unfortunate experience of reading essays where every second sentence is a quotation. These are some of the most awful essays to read. Quotations break the flow of your narrative, and do not show your understanding of the work that you are discussing.
For example, I can copy and paste a paragraph from a Spanish text, and it can be grammatically perfect, but that does not mean that I have any idea what I have “written”. The same is true for quoting from English words. If you are going to provide a quotation (because I can’t actually stop you from doing so) then it MUST be accompanied by contextualisation and an explanation.
Something else to consider is that if your marker wanted to read word-for-word what other people had to say, s/he would read their texts. What markers want to see in your work is your work, and that you are able to integrate information from different sources and convey this information in a logical and coherent way. My general argument would be that unless you read something that is so perfectly written that it brings tears (of joy) to your eyes, you should always avoid quotations.
Note: This does not apply to academic assignments that are based on specific texts (this is usually the case in, for example, English Literature essays). In these assignments you are usually required to provide textual support for your argument.