Of course there are necessary illusions. Without them, humans would find it very difficult to maintain their sanity. Without them, the human intellect would also be very homogeneous. Illusions give us the ability to be innovative and creative when solving difficult problems related to reality. In other words, illusions are necessary to life.
Some portion of people - in many cases a large portion, honestly - will not accept things, regardless of the evidence in their favor, if something cosmetic or superficial makes them averse to them. In situations such as GMOs or climate issues, necessary illusions to garner public support help the overall goal.
I do think that there are necessary illusions. Some people need illusions of various things to help them get through life. Those are necessary illusions to me. For example, some people need to portray the illusion that they have a perfect family life. That helps them get through life. That illusion is necessary for them.
I do not believe there are necessary illusions. I believe any of the illusions performed by humans are either for entertainment purpose or a method to cover up something, someone is doing wrong. Illusions beyond our capability to understand them or most importantly our ability to recognize them, may however be very necessary.
There is no such thing as a necessary illusion. As someone who believes in trying to maintain honestly on all accounts, illusion is not something I can support. Illusions may make life easier in the here and now but, if something is not real and genuine, it is not honest and therefore no good can come from it.