Sometimes they can raise your bobbin up just a hair...enough for your thread to catch on and cause a jam. I have this issue with drop in bobbins.
Honestly I have never needed bobbin washers with plastic drop in bobbins. I do a lot of heirloom quilting on a simple inexpensive Brother from Walmart among other machines. They can be helpful for metal bobbins in metal cases that don't already have bobbin springs.
