Implementing ITIL is a lot like cooking: It has accurate measurements and directions, but a good company (much like a good chef) knows how to fine tune the recipe to fit what is needed or what works best ““ without completely modifying the original directions. This post from ITSM Professor uses this comparison to help illustrate best practices in ITIL:
Adjusting and adapting ITIL must be thought of in the same way. We may not like what the best practices tell us we must do or even how we must do it. However, if we decide we are going to rewrite the processes or change the way we design services or how we determine our Configuration Items, then we should not expect the results intended with the best practices. It is a classic case of “wanting our cake and being able to eat it too.” We cannot remake best practices to suit our desires and expect optimal results. We cannot effectively manage and control changes using a demand management process.
The post explains the dangers: if organizations aren't willing to fit what is required for ITIL, then they are not likely to experience the benefits of using ITIL at all. After all, it's called ITIL adoption, not ITIL pick and choose.