Hi John.
I agree that Decision Makers (DM'S) represent the most important segment and you clearly want to regularly assess their sentiment and identify ways to improve the service experience. However, other customer segments (end users, influencers, etc.) certainly have the ability to sway the opinion of a Decision Maker. As the name implies, they can "influence" a decision.
It is for that reason that we recommend inviting all those contacts that have the potential to influence the relationship to participate in the survey. We want to know what everyone thinks. If we aren't serving a particular segment well, that knowledge should drive action. We have some clients where the Administrative Assistant is a prominent user of the software. If they are frustrated, they could complain to their boss who very well may be the Decision Maker.
Gathering feedback from all constituents does require you consider how your Net Promoter Score will be calculated. We have a couple of different methodologies - all well thought out based on several business considerations which dictate the approach we use for that client. For some, it's a simple - count all who responded. For others, we count the Decision Maker score only (or the more influential contact if the DM didn't respond) to calculate the company score. That's not to say that feedback from other contacts isn't important. It is considered heavily when identifying process improvement decisions, resource allocation, etc.
However, to your point, we don't want several responses from possible influencers to skew the data/score. We will also calculate Net Promoter Scores for various user groups and segments. This allows you to see if one segment feels your product / service is terrific but DM's think otherwise (for example).
Hope this helps.
Evan Klein
Satrix Solutions
Net Promoter Loyalty Partner