.. but, perhaps, a little more seriously ..
(a) the OEM's stuff may not be perfect but it's supported and greatly simplifies the operator's workload and documentation QA overhead.
(b) pinching someone else's stuff, generally, is cheap but silly as you also pinch any inappropriate stuff within their documentation. Either you have to amend that or wear it ?
(c) at the Enquiry .. perhaps you have an advantage by relying on the OEM stuff ?
(d) having worked with/for operators who varied from do-it-yourself to straight OEM, I know which makes more sense to me.
(e) SOP differences ? .. sometimes necessary and appropriate but, often, just the product of ego. How many of us have worked for airlines where a long term Type suddenly gets a major documentation makeover coincident with the appointment of a new Fleet Boss ? I fondly recall, long ago, an ex-Fleet Boss, when visited for a beer at home by the then Fleet Boss, had the embarrassing situation of the current Ops Manual's being used as his front door stop ... tells a story, I think ?