They are still needed with older TIUs where the signal is weaker. I believe it is general said to try disconnecting them and any bulbs when you install the newer versions with L on the bottom and the USB port. Someone may still find them beneficial in areas where the signal is doubling back. I also believe that can be caused by double feeding a block and should be corrected anyways.
I believe most other troubles with an L version, are probably caused by having signal wreckers like any cars with caps and no chokes inside. It is tough to guess at each layout without having any facts. I found upgrading to the L version, took away signal issues with all my 3 layouts, G & O. I put a single channel of an L where 2 were needed, and the Lionel tank cars with sound tipped the scales on me. Signal dropped way off when they entered the blocks the engines were in. So the L is better, but not fool proof.