I think the main argument against a book written in multiple languages is aesthetic.
And we are not talking about books, like War and Peace, that have small parts in another language, or quote texts in the original language, but books truly written in two or more languages.
Such a book can be easily imagined, when you listen to people who grow up multilingually: for example, here, in Germany, there is a large community of adolescents of Turkish origin. These youths often speak Turkish at home and German in school and with their non-Turkish friends. When you listen to two Turkish kids on the bus, they communicate in a Pidgin mix of German and Turkish. They do so, because some things are better or more easily said in on language than the other, so they switch to whatever comes most easily or sound most cool, or whatever.
From this example – and maybe you have witnessed such a conversation yourself – you can easily imagine a book being written in the same way, with every section or sentence or part of sentence being written in the language that either best conveys the authors intention or sounds best or comes most easily to him.
But why is that a problem, aesthetically, if your readers are fluent in all the used languages? Because the reasons given are not acceptable reasons from an aesthetic perspective.
Writing is a form of art. The assumption is that not everyone can write at a level worthy of publication, or that at least they have to learn it. It takes training to achieve a level of mastery that results in elegant and pleasing sentences. And it takes talent and/or diligent training over many years to enter the ranks of the literary greats.
The idea to write (in one or more languages) as you find it most easy, contradicts the idea of art. Art is based on the idea of ingenuity: of fighting hard, and against the inherent obstactles of a language, for a beautiful text. Writing in multiple languages because that way you can better express yourself means that you take it easy, that you are lazy, that you did not master language, but that it masters you.