As a recruiting practitioner I really see no value personally or professionally in being a SHRM member or gaining a certificate. Unless someone could give me a really good reason to the contrary.

• show you know the most current principles and core practices of HR management
• become more marketable when you compete for top HR positions
• raise your professional confidence among your staff and your peers 

• certified HR professionals earn 13.2% higher salary than their non-certified peers
• organizations with certified HR professionals have 12.2% greater retention of high performing employees
• organizations with certified HR professionals have 14.6% less EEOC complaints than organizations led by non-certified HR leaders

With the increasing popularity of the SHRM certification program as well as the proliferation of other similar certifications, it is time to determine (1) what certification measures and how well it does it; and (2) what difference having certification makes to individuals, to organizations, and to the HR profession.

The figure depicts three important elements: (a) time, (b) macro-level effects, and (c) micro-level effects. First, the starting time begins with the implementation of the organizational staffing practice of using HR certification as a selection tool. This staffing practice represents a contextual (top-down) effect on the organization's individual KSAOs because all potential HR professional employees will be recruited using the HR certification predictor. HR certification is used as a selection tool because it measures human resource management knowledge

• Proposition 2: Individuals with undergraduate or master's degrees in human resource management or related disciplines will not benefit as much (increased hiring probability, higher salaries, and faster promotion rates) from an HR certification compared to individuals without such degrees
• Proposition 3: As assessed by longitudinal research, we predict that individuals with undergraduate or master's degrees in human resource management or master's degrees in business administration will obtain more C-suite positions and perform more effectively in these roles compared to individuals who have certifications such as SHRM's PHR or SPHR
• Proposition 4: Using HR certification as a selection tool will be positively associated to individuals' HR management knowledge
• Proposition 5: Certified HR professionals will perform better (i.e. effectively diagnose and resolve HR problems, design and implement HR programs aligned with organizational objectives, etc.) on the job than do non-certified HR professionals
• Proposition 6: A greater proportion of certified HR professionals who have shared HR knowledge in an HR department will lead to higher unit-level performance
• Proposition 11: A greater proportion of certified HR professionals in the HR function will lead to more favorable perceptions by multiple constituents regarding the value-added contribution of the HR function in the organization and thereby enhance HR department reputation
• Proposition 13: A greater proportion of certified HR professionals in an HR department will lead to higher HR department effectiveness

In order to test Proposition 2, data would need to be collected from a single huge organization with a large enough population of HR professionals who are covered by the same compensation plan to test the proposition. You would need archival data of salary trends, certification status, educational achievements, promotions, performance data, and so forth to comparatively test for the differences in salary and advancement.  A study of just HR professionals, outside of the archival factors mentioned many not provide accurate or generalizable information.  Titles may or may not be comparable across organizations so unless the data comes from a single organization where title, responsibility and other variables can be controlled, the results may not reveal anything meaningful.