<p>
This is pretty unsound advice. Of course the post-doc or grad student you directly worked with will be able to offer more detail on your capabilities and successes than the PI you interact infrequently with. But having a non-professor name (and only a non-professor name) attached to the letter makes it much less meaningful, since the recommendation is not coming from an established researcher and peer to the admissions committee members. Most PIs are willing to have the student’s post-doc or grad student mentor contribute to or draft the LOR, and then the PI will co-sign it. This is much more preferable to having only the direct mentor write a letter. It’s also preferable to sending two recs from the same lab, which are bound to say redundant things about the applicant.</p>