Conventional advice to legal writers underplays the advantages of complex sentences. Well-written complex sentences are not only more Euphonious than strings of short, choppy, simple ones but also afford additional hierarchical structure. The subordinate clauses of complex sentences serve a function analogous to footnotes, but unlike footnotes, subordinate clauses are not subtextual, as they do not incorporate matter less important than ordinary text, instead structuring the text itself. Complex sentences convey information about the relationship between clauses. Using simple sentences instead means either omitting the relational information, undermining Clarity, or including the relational matter discursively, undermining Concision.
The advice to use short and simple sentences includes particles of truth. One is that interclause relationships have diminished importance for some legal-writing purposes. Where the writer refers to clearly dispositive authority in a brief supporting a procedural motion, too much attention to interrelations may distract rather than clarify. Another particle of truth in the short-sentence advice is that shortening and simplifying sentence structure will improve many lawyers' writing. Short, simple sentences, easier to write than long, complex ones, can help avoid excess verbiage, grammatical error, and unintelligibility.
For lawyers who are not writers by choice, shortening and simplifying sentences is often good practical advice, but you should expect more from a ghostwriter.