If you have a validation rule that requires this picklist field to be populated under certain circumstances, then the rule will require the user to enter something if that field is null. So it sounds like you need that field to be null *before* the user enters information, so that it will force them to enter a value when they save (if necessary).

Your picklist has values of "Yes" and "No". However, it sounds like you're saying that there is a point where Pipeline is essentially not determined or not committed yet (according to your criteria), and this would be equivalent to that picklist being null. You really have three values, then: "Yes", "No" and null.

My idea: 

 - have this field default to null.
 - use a before update/before insert Apex trigger to set this field to Null unless it has already been set to "Yes", or the commitment criteria have been met and it has been set to "No". 
 - use a validation rule requiring that this field be populated if the commitment criteria have been met.
 - In reporting, group records with Pipeline = null and Pipeline = "No" together as non-pipeline records. This could be done through a formula field.

Edit: that one won't work, because there are multiple "commitment" points, based on Stage. Every time the Stage changes, after a certain point in the sales process, the user needs to decide to select "No" or "Yes". 

New idea:

 - Replace "No" with multiple "No" choices: "Stage 3, No Pipeline", "Stage 4, No Pipeline" and so on. 
 - Create a picklist dependency between Stage and Pipeline, so that at Stage 3 you only have the choices of "Yes" and "Stage 3, No Pipeline"; at Stage 4, you only have "Yes" and "Stage 4, No Pipeline". When the user changes the Stage from 3 to 4, the Pipeline picklist will automatically clear itself on the page if it was "Stage 3, No Pipeline" before. If it was "Yes" before, it will stay "Yes".
 - Your validation rule requires setting the Pipeline field if Stage is one of 3,4,5,6
 - Reporting can lump all the No Pipeline Opportunities together, again through a formula field