The following are six reasons to have a SharePoint partner:
1) Reduce expenses by having someone who can offer a SharePoint delivery model for what is needed
2) Have someone who can help with project methodologies for SharePoint
3) Have someone who can help with an end-to-end SharePoint services portfolio
4) Have someone to help with a standardized approach and rapid adoption of best practices of SharePoint
5) Have someone to help with faster responses to SharePoint business requirements by ramping up/down.
6) Have someone to help with extreme SharePoint offshoring if applicable
