In many ERP implementation projects, we run into a collision between approach used by vendors versus the one preferred by the client.
Pretty much most quality ERP vendors offer a methodology which is based on Project Management Institute's (PMI) principles. Each vendor has added its own touch to make it a better fit for the type of clients they serve and also to better link their professional service revenue generation to a milestone based approach.
Clients on the other hand, feel they have made a significant investment in software, they own it, they know their business the best, so they want to control the process and do it their way!
Both approaches, if used in absolute terms, are destined for project failures or major cost and time escalations. Vendor methodologies, although good for vendors to standardize and scale their businesses, fail to realize differences within various organizational systems, on issues such as people, culture, processes, etc. Customer driven methodologies, on the other hand, will setup the company for a journey into wild territories since most companies go through an ERP project once every 7-10 years at best and have very little experience with intricacies of such projects.
There needs to be a compromise. Without a middle of the road approach, we would be lacking a methodology to count on for planning and minimizing risks and by not taking client's needs into consideration and making adjustments to our approach, we will risk alienating clients project team early on in the process and setting ourselves up for failure.