Admittedly I applied 5 years ago, but I did not have a non-sci letter due to various circumstances (and I've reviewed plenty of apps w/o non-sci letters). In fact, all my letters were pretty much... science. Ultimately was accepted to several schools where I did not fit their LoR requirements, so I don't think it's a complete deal-breaker.
The most informative LoR portfolio likely comes from a wide breadth of people – professors, employers, PIs, nonprofit leaders, etc. I think the non-science LoR is an effort to get someone who can attest to a different skillset than your usual pre-med science req letters. Ask people who you think would be eager and excited about your application; those are the ones that want to be read!
I'd say a lacklustre letter is more harmful than the absence of one. Often times the professorial letters can be the least informative (or I am sure mine were at any rate: "Hella Nutella was a student in my bio class. She did good on exams. Did not seem to have glaring personality defects. Once ate an entire wheel of Laughing Cow cheese during a midterm").
