Recommendation letter from a PhD-to-be? I am applying for Master programs in Educational and Clinical Psychology. And I am in the process of choosing people for writing my recommendation letters. I have a BA in Psychology and I wasn't much involved in research - except when I did my year aboard, I participated in a research project launched by a teaching assistant who was then a PhD student at that college. We conducted the experiments together, collected and analyzed the results. In the end, he supervised my Research Project Thesis. 

It was a valuable hands-on research experience for me. And I am thinking about asking him to write my recommendation letter. However, though he is still actively involved in several research, he hasn't yet obtained his PhD degree (he will in the coming year). So I am concerned that a recommendation letter written by him would look nice, but at the same time it would lack some sort of legitimacy and affect my chance of being shortlisted for further interviews.  

So my question is - would a recommendation letter written by a non-PhD referee carry less weight and even not taken seriously? Should I ask this person to write the recommendation letter for me?

Thanks for viewing my question and I look forward to your answers. Thank you for your help!
Typically reference letters should come from faculty, as you implied.  If you were supervised by a graduate student, ask the graduate student's faculty advisor to write the letter.  Very likely the faculty member will ask the graduate student for assistance in writing the letter.  This will get you the legitimacy and the content you need.  It can be awkward because faculty need to create the impression they wrote the letter themselves, but you can finesse it.  When I was a PhD student supervising undergraduates I just told them that I would ghostwrite the professor's letter.