A query came up on another URB board I moderate: http://pub41.ezboard.com/fthedudepitfrm37

"I asked this over at Thomasik but they're puzzled too!
Can you help?......OK, this may qualify as a silly question but....
I use Weichs and every set I bought has a small piece of tubing slipped over the D string.
I've asked just about everyone (including the UK distributor) what the purpose of this mystery component is but no-one seems to know.
I've tried using it at the string/bridge contact point which seems the only logical application but all it did was to cut the sustain.
There has to be a reason for it to be supplied...but what?
Your answer eagerly awaited!! "

I remember using the tubing on violin E and A strings sometimes, which cut down on the harshness a bit, but a bass D?? Anyone??

Hey Bob, on a set of Spirocore Orchestras that I have that same piece of black tubing came on the D string. I had no idea what it was for either. I just threw it out.

i heard your supposed to put it where the string touches the tailpiece so it doesn't wear through and make a groove. seems weird that if that's what it's used for they would only supply it for the D.

Got an official response from Thomastik on the other board:

Re: Plastic on Thomastik
I love learning new things every day! Here's today's funfact:
The tube on the D string is used for reaching a "less metallic" sound. In fact it cuts off high frequencies, this might reduce the sustain a little, especially the sustain of higher harmonics.
Most of the players do not use this tube or they install it just because they think they should. It is not needed. Sorry for taking so long getting back to you but my knowledge is a bit limited when it comes to the bowed line of TI. I'm learning, though...

This info comes straight from T.I. HQ in Vienna...

My best,
Kevin Reynolds

-
I mentioned recalling they were on the two high strings of my violins, used to make the sound less harsh, which sounds like the same on bass according to TI. I wonder why not on the G, too??