They use an analogy that someone would not go into a restaurant and tell a chef how to make food. I believe djing a party is a wee bit different. As djs we play for the people. The best requests are the ones that go with the genre that the dj is playing. For example, if the dj is playing Bell Biv Devoe and a guest suggests throwing in En Vogue. That is applauded. Usually I put in on my prepare list and throw in as soon as I can. If it isn’t the right genre I can still put it on my prepare list and can throw it on when its appropriate. If they ask for a song I already play, then I ask them if there is another song I can play for them or I give them suggestions they can choose from.
When I play at an event, I use the playlist given by the client and I will include music suggested by the guests. I will happily play the suggestion. If every guest came with a suggestion, I would add it to my list. It can be a little much when the same guest keeps on asking for more and more songs and the event becomes more about that guest than the rest of the guests of the party. As a dj, I never ask another dj to play my song since I know its difficult to dj a party and maintain the dancing vibe. Sometimes djs get in a zone and playing hit after hit and the floor is jamming. Be aware the wrong request can kill the party and the concentration of the dj.