The guidance here indicates the DisableMethodInvocation SHOULD be used with quality measures to prevent method-style invocation. That decision was made quite a while ago, before the introduction of fluent functions in CQL, and was meant to prevent unexpected usage of FHIRPath functions, which were "fluent", but only for the set understood by the compiler (as in, pre-defined in the FHIRPath spec). Now that fluent functions are a feature of the language, this decision should be revisited, because the benefits of fluent functions for readability are clear.